Add __len__ method to ExistsType

[ArVar]

Adds len method to the ExistType like in CategoricalType. Otherwise the ExistType will use the __len__ method of Variable:

    def __len__(self) -> int:
        return 1

and higher_vars won't be expanded correctly in datamodel.py:

        for variable in self.field_variables:
            if len(variable) == 1:
                columns.append(variable)
            elif len(variable) > 1:
                assert hasattr(variable, "higher_vars")
                columns.extend(variable.higher_vars)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.78%. Comparing base (f7e131e) to head (2a558db).
Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1199      +/-   ##
==========================================
+ Coverage   73.57%   74.78%   +1.21%     
==========================================
  Files          29       29              
  Lines        2293     2217      -76     
==========================================
- Hits         1687     1658      -29     
+ Misses        606      559      -47     

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[ArVar]

Hi @fgregg, are the codecov rules a bit too strict?
I haven't seen any test for __len__ in the repo.