ENH: define constraints by strings with wild cards

[josef-pkt]

a general idea

see also related, more general issue #4361

we need constraints or contrast matrices in several places,

• original definition is in t_test and similar.
• test_wald_terms uses some string matching to construct constraints
• fit_constrained uses the same pattern, IIRC
• penalized fit needs restriction matrices to define stochastic constraints

proposal:

• I think we can create a standalone function following the t_test pattern.
• It would be convenient to allow for wild cards as in Stata,
  e.g. "region*" for all parameters that encode a categorical variable
• The latter could by also used with sparse matrices, e.g. as in variance components in mixed models, or variation on sparse OLS and sparse penalized models.

get_constraints(constraints_string, param_names)
or some similar name

possible extension to automatic constraint patterns
e.g. all pairwise #4365