source_max etc. -> source_use_max etc.; merge purchased and units

[brynpickering]

Fixes #451

Summary of changes in this pull request:

• source_max/sink_max etc. parameters to source_use_max/sink_use_max etc.
• units and purchased merged into purchased_units. Setting purchased_units_max to 1 creates the same result as the former purchased binary variable.
• Moved some milp constraints to base from one of the example custom math YAMLs (available_flow_cap). This allows a user to define either flow_cap_per_unit or flow_cap_max for a technology with integer capacity units. This way the time-varying integer variable operating_units can be applied to the case where purchased_units is being used as a binary variable and flow_cap remains continuous, where it used to only apply with flow_cap being fixed to a multiple of purchased_units.

Reviewer checklist:

• [ ] Test(s) added to cover contribution
• [ ] Documentation updated
• [ ] Changelog updated
• [ ] Coverage maintained or improved

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4e70191) 95.00% compared to head (c7cf81f) 95.14%. Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #533 +/- ## ========================================== + Coverage 95.00% 95.14% +0.14% ========================================== Files 24 24 Lines 3301 3298 -3 Branches 705 651 -54 ========================================== + Hits 3136 3138 +2 + Misses 96 92 -4 + Partials 69 68 -1 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/533?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project) | Coverage Δ | | |---|---|---| | [src/calliope/backend/backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/533?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvYmFja2VuZF9tb2RlbC5weQ==) | `97.65% <100.00%> (+2.23%)` | :arrow_up: | | [src/calliope/backend/expression\_parser.py](https://app.codecov.io/gh/calliope-project/calliope/pull/533?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvZXhwcmVzc2lvbl9wYXJzZXIucHk=) | `97.14% <ø> (ø)` | | | [src/calliope/preprocess/time.py](https://app.codecov.io/gh/calliope-project/calliope/pull/533?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL3ByZXByb2Nlc3MvdGltZS5weQ==) | `88.57% <ø> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/calliope-project/calliope/pull/533/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project)