Revisit signature of model methods representing terms in PDEs

[vlipovac]

There are inconsistencies in the signature of respective methods, mostly regarding the usage of list[pp.Grid], pp.SubdomainOrBoundaries etc.

See this comment for example.

Fix:

1. Go through models/*.py and check the signature of methods representing terms in some PDE or other equation.
2. If the term appears possibly on the boundary, extend to pp.SubdomainOrBoundaries for example.

The challenge lies within the nested nature of functions. F.e. a density function might call 2-3 functions internally, which also require the same signature, but overall it should be a quick fix.