Prevent underscores from being removed in dim names

[tomicapretto]

The dimension names of the groups specific effects end with __expr_dim and __factor_dim.

For example, if the term is (categorical | group) then the dimension names are categorical__expr_dim and group__factor_dim.

When using aliases, we were replacing the term name with the alias, but we were accidentally removing the "__", so dimnames would turn out to be aliasfactor_dim. This PR fixes that so the dimname is alias__factor_dim.

[codecov-commenter]

Codecov Report

Merging #664 (3d1d85f) into main (341966f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #664   +/-   ##
=======================================
  Coverage   87.62%   87.62%           
=======================================
  Files          40       40           
  Lines        2650     2650           
=======================================
  Hits         2322     2322           
  Misses        328      328           

Impacted Files	Coverage Δ
bambi/backend/terms.py	96.44% <100.00%> (ø)
bambi/families/multivariate.py	98.33% <100.00%> (ø)
bambi/families/univariate.py	90.37% <100.00%> (ø)

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