Replaced Model.set_equation(lhs, rhs) with Model.remove_equation(equation)

[MichaelClerx]

Fixes #118.

As discussed in #118 I removed set_equation(lhs, rhs) and instead added remove_equation(eq) which can be used together with add_equation(eq) to achieve the same effect.

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:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #171 into develop will decrease coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #171      +/-   ##
===========================================
- Coverage    95.92%   95.89%   -0.04%     
===========================================
  Files            6        6              
  Lines          883      876       -7     
  Branches       186      182       -4     
===========================================
- Hits           847      840       -7     
  Misses          19       19              
  Partials        17       17

Impacted Files	Coverage Δ
cellmlmanip/model.py	99.62% <100%> (-0.01%)	⬇️

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[MauriceHendrix]

Looks pretty good to me.

[jonc125]

Looks fine. I suggested a possible simplification, but nothing dramatic.