Rename DataPostprocessor functions.

Rename DataPostprocessor functions.

In #3333, I added virtual functions to DataPostprocessor with the same name as the existing
functions. This leads to issues where we get a warning in every derived class that only
overloads one of these functions, because that hides the other function. This is, well,
suboptimal.

This patch is therefore a redo of my earlier attempt in which I continue to deprecate
the old functions, but the new functions have a different name. I think they also
have a better name (for a discussion of the naming, see
https://github.com/geodynamics/aspect/issues/1284 ). The different names avoid the
problem of getting the warning and should therefore lead to less discontent. They
also avoid the need to try to work around the warnings using 'using' declarations, like
in #3528.