Remove pointless indexsets.tabulate parameter include_data

[glatterf42]

Partly closes #137: this PR removes the include_data parameter, but doesn't add any way to check the length of the indexset.data. For that you still have to get the indexset somehow and access the attribute directly.

Please note that I did quickly check if including len(data) would easily be possible. I don't think it is because indexset.data is a @property on the DB model class at the moment, so not loaded automatically when using tabulate. That's because base.tabulate() uses pd.read_sql(), which only reads in the columns of the returned data model (not relationships or attributes built on them).
In contrast, e.g. base.list() returns a list of data models and their attributes can be accessed normally.
Thus, it seems to me that there's no quick way to change that for indexset.tabulate(), though it can definitely be done; either through rewriting base.tabulate() (think: SELECT all resulting rows first, load their .data, then turn into pd.DataFrame) or by accessing the DB twice (think: use base.tabulate()as usual, then use e.g.base.list()to get the.datadata and put it into a new column). Neither of those options seems desirable to me at the moment and I don't think it's necessary to comply with themessage_ix` API, so i would ignore that part of #137 for now (at least, if not forever).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.9%. Comparing base (c18ed42) to head (79b2c14).

Additional details and impacted files

@@                      Coverage Diff                      @@
##           enh/professional-grade-mypy    #138     +/-   ##
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- Coverage                         86.9%   86.9%   -0.1%     
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  Files                              228     228             
  Lines                             8141    8135      -6     
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- Hits                              7081    7075      -6     
  Misses                            1060    1060             

Files with missing lines	Coverage Δ
ixmp4/core/optimization/indexset.py	92.6% <100.0%> (ø)
ixmp4/data/abstract/optimization/indexset.py	80.6% <100.0%> (ø)
ixmp4/data/api/optimization/indexset.py	97.6% <100.0%> (ø)
ixmp4/data/db/base.py	91.8% <ø> (-0.1%)	⬇️
ixmp4/data/db/optimization/indexset/repository.py	98.1% <100.0%> (-0.2%)	⬇️
ixmp4/server/rest/optimization/indexset.py	100.0% <ø> (ø)