mjo/cone/decomposition.py: add tests, fix the edges of the edges

mjo/cone/decomposition.py: add tests, fix the edges of the edges

A miscellaneous selection:

  * The vertex numbering was off because pivots() renumbers them.
  * Updated the docs to fix some incorrect claims.
  * Added lots of new tests.
  * Special case for irreducible cones to return K itself.
  * Use subspace_with_basis() instead of span() to preserve
    our user basis.

Tests now pass, and the documentation is converging upon the truth.