Feat/Richer cmd output for listing commands

[germa89]

Related to #735

Basic

It implements a class for listing command output which has three methods: to_list, to_array and to_dataframe (following Pandas convention).

Idea

It seems many of the listing commands are based in the same MAPDL fortran function, so in principle, making this CommandOutputListing class work with one of them, should make it work with all (most?) of them.

TODOs

• Adding base class
• Adding allowable commands
• Adding unit tests
• Add documentation. Cancelled in favor of a separated PR.

Dependencies

This PRs needs for #791

Testing

It will be limited to get output from the correspondent methods.

[akaszynski]

Thanks for fixing the lazy import. Merge as soon as checks pass.