Draft: make the generic functions consistent

Addresses #232 (closed), #205 (closed),

TODOs:

• fix docstrings
• more tests
• cleanup
• merge develop
• prepare description
• bring back isflagged

Summary

In order to make the generic functions consistent with all other test functions (see #205 (closed) for details on the consistency issues) and still keep the useful, this MR basically implements a non-scalar field parameters as outlined in #205 (closed). With these changes in place, we now support:

• univariate functions: iterative execution of functions as described in points 1, 2, 3, 4 in the table in #205 (closed)
• multivariate functions:

There are a couple of major changes:

• The decorators processing and flagging got a new parameter called multi.
• We pass another kwargs argument to all functions, namely target.
• Flags.__getitem__ now supports Sequences to slice a Flag object.