adding unit conversion functions

[skeating]

Fixes #77

Description

Adding functions necessary for unit conversion.

• take a variable and unit and convert all relevant values/equations in model to reflect new unit on variable
• add_input function
• add_output function
• model missing units - MOVED to issue Add a custom unit to registry as a Unit object #192
• arguments as strings/objects
• review arg checking
• refactor is_output to not be in API
• test state symbols are correct after an input is added
• test substitution of rhs deriv recurses into sub expressions
• add helper function for getting unique name
• check _convert_variable_instance need not replace deriv for output (step 4)
• formatting of docstring examples
• change function name/args/add enum

Motivation and Context

Fixes #77

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@bd8411f). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #175   +/-   ##
==========================================
  Coverage           ?   96.14%           
==========================================
  Files              ?        6           
  Lines              ?      987           
  Branches           ?      208           
==========================================
  Hits               ?      949           
  Misses             ?       17           
  Partials           ?       21

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bd8411f...6cc0e0c. Read the comment docs.

[MichaelClerx]

This is looking good!

[skeating]

Requesting review of whats here.
Questions:

1. how do I distinguish between input/output if I wanted to create a generic convert(some_variable, some_units) and it figured out which to call ?
2. should I make it so that arguments are either string/DummyVariable and string/Unit object ?
3. any ideas why appveyor is failing - tests pass locally
4. I know I need to deal with case where the model does not have the required units in its registry

[skeating]

Asking for review of this as it stands. With hope to merge with following questions:

1. Do I need to fiddle and get appveyor to pass

2. I could change current convert_variable so that the unit argument could also be a name/attributes pair which would allow it to be added if necessary while I work on Add a custom unit to registry as a Unit object #192

Naming is sorted and Michael is away for a month

[jonc125]

Asking for review of this as it stands. With hope to merge with following questions:

1. Do I need to fiddle and get appveyor to pass

No, sympy 1.5 support should be addressed in a separate PR.

2. I could change current convert_variable so that the unit argument could also be a name/attributes pair which would allow it to be added if necessary while I work on #192

I don't think we want to do this.

[skeating]

Sorry @jonc125 just saw your post on slack. Code at present works if unit to be converted-to is in same registry as converted-from so it covers your suggestion. Weblab-fc would be place to take care of protocol/model using same registry and that I'll cover with taking over Michael's issue. I'll leave the other issue open and think a bit more.

[jonc125]

With two exceptions I think these are just cosmetic/documentation comments. The more substantive items are:

1. After seeing the code, I think it'll actually make our lives easier to say convert_variable has to be passed a VariableDummy.
2. When converting we ought to delete the initial_value from the original variable if it's moved over, as it gets defined by an equation instead.

[jonc125]

Looks good. Just one typo and one minor fix spotted!

[jonc125]

s/b/be/