Enforce minimum service factor limits

[alexdewar]

Description

I've fixed #320 by introducing a check in the minimum_service constraint function that all minimum service factors are >=0 and <= 1 and I've also amended the spurious tests accordingly.

The only thing is that I'm wondering whether this is a bit late to raise an error. Should the check be in some earlier parsing step (either in addition or instead)?

While I was reading through the docstring for muse.constraints I noticed some typos and fixed those while I was at it.

Fixes #320.

Type of change

• [ ] New feature (non-breaking change which adds functionality)
• [ ] Optimization (non-breaking, back-end change that speeds up the code)
• [x] 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
• [x] Tests added that prove fix is effective or that feature works

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.09%. Comparing base (89754e6) to head (c7a0101).

Files	Patch %	Lines
src/muse/constraints.py	33.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #325 +/- ## =========================================== - Coverage 71.11% 71.09% -0.02% =========================================== Files 44 44 Lines 5809 5812 +3 Branches 1147 1148 +1 =========================================== + Hits 4131 4132 +1 - Misses 1358 1359 +1 - Partials 320 321 +1 ```

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