Allow None for XSEL commands

[germa89]

Description of the feature

It will be nice to allow Pythonic None as a valid value for the Type_ argument, so we can do:

mapdl.nsel(None) # equal to mapdl.nsel("NONE")

Steps for implementing the feature

Internally it is just:

if type_ is None:
    type_ = "NONE"

This can be added to a wrapped function.

Useful links and references

NA