Implement character and representation theory

[fingolfin]

For starters, move the character table code from examples/GroupCharacters.jl to regular interface.

This is @ThomasBreuer code and I see no fundamental obstacle? Of course may wish to adjust the interface some

UPDATE: a wishlist of things from some old notes of mine; note that of course not all of these need to be there from the start

• characters and character tables should exist as Oscar objects
  • characters should be applicable as functions, but also can be added/scaled/etc. like vectors
  • integrate with Oscar number theory
    • conversion between GAP cyclotomics Oscar number fields in both directions (see PR #710)
    • ...
  • nice printing
    • to HTML and to LaTeX, for use in Jupyter notebooks
  • Schur indices
• representations...
  • should these be a dedicate object type? and/or a type of homomorphisms? or...?
  • possibly integrate the functionality of the GAP package repsn from the start, to go from characters to reps and back
  • ...
• G-modules (over any field)
  • operations with these (sums, tensor products), ...
  • compute (co)homology of these (by integrating with Claus' existing code)
  • ...
• conversion between characters, representations, G-modules
• ...

[fingolfin]

I am not sure we need a separate "representation" type at this point, but some ideas for things that should be possible, and should ideally be illustrated in the manual with examples (be it in docstrings, or the general text, or ...):

• map/convert/... from a matrix group to a G-module and back
• map/convert/... G-modules to characters
• make sure Meataxe functionality is hooked up, at least in cases where we support it (finite fields?) e.g. by calling into the corresponding GAP functionality (I think @fieker once mention there is also already something available in this regard on the Julia side? If so, where/what?)
• beyond that it may generally just be nice to be able to convert GAP GModules to Oscar G-modules (and back, where possible/suitable)
• and of course the things that are already listed above