Create PyMC-specific RV Ops that overwrite _print_name

[larryshamalama]

Currently, some RV's _print_names contain the abbreviated version of the random variable Op rather than its full name, e.g. Normal, Cauchy, etc. When it come to displaying them in Graphviz, we resort to its class name via some_op.__class__.__name__, which, to my understanding, is a bit hackish. A solution would look like the following:

PyMCNormalRV(NormalRV):
    _print_name = ("Normal", ...)

pymc_normal = PyMCNormalRV()

class Normal(Continuous):
    rv_op = pymc_normal

In particular, with the newly added SymbolicRVs, this could be helpful (see WIP PR #6149).

CC @ricardoV94 @michaelosthege