Fix issues with costs outputs

[tsmbland]

Description

Fixes a couple of issues with the costs outputs in outputs/mca.py. The first of which emerged after we upgraded pandas/xarray, the second was an issue before (but was never caught).

Changes:

• Use multiindex_to_coords to fix the ValueError: cannot insert hour, already exists error
• Change "new" to "newcapa", which fixes a bug where the LCOE and EAC outputs were empty when only using new agents

Remaining problems with the costs outputs:

• Emission costs output doesn't work #421
• Fuel costs output gives zero for every value #419
• Better unit tests for model outputs #422

Fixes #383

Type of change

Please add a line in the relevant section of
CHANGELOG.md to
document the change (include PR #) - note reverse order of PR #s.

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python -m sphinx -b html docs docs/build

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 24 lines in your changes missing coverage. Please review.

Project coverage is 71.35%. Comparing base (692c0e8) to head (769c802).

Files	Patch %	Lines
src/muse/outputs/mca.py	0.00%	24 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #420      +/-   ##
===========================================
- Coverage    71.38%   71.35%   -0.04%     
===========================================
  Files           44       44              
  Lines         5893     5893              
  Branches      1160     1156       -4     
===========================================
- Hits          4207     4205       -2     
- Misses        1366     1367       +1     
- Partials       320      321       +1     

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[dalonsoa]

Looks good. The code is way more readable now, in addition to fixing the bugs, which is always good news.

[dalonsoa]

This looks ok, but it is doing something different than before. I assume this is correct, but I'm not sure why it is.

[dalonsoa]

ah, ok, you are combining it with what it was being done afterwards. OK, looks fine.

[alexdewar]

FYI @tsmbland I'm just going through your PRs now

[alexdewar]

I've only given it a quick scan, but LGTM!