RFC: Expands array doc

[jiahao]

The docs are currently very sparse (sic) on the matrix factorization objects, the special matrix types, and the relationship between dense matrix methods and their sparse counterparts.

To make a first stab at addressing these lacunae, this pull request provides:

• Table of available matrix factorizations
• Tables of available special matrix types and availability of optimized methods
• Table of correspondence between dense and sparse matrix methods
• Minor speeling fixes

[ViralBShah]

Really nice!

[ViralBShah]

We really need to combine Arrays, Matrix factorizations, Special matrices, and Sparse matrices into one chapter in the manual. Currently, they are 4 top-level chapters!

[jiahao]

Oops, I think this is a formatting error with my headings; they are all in the same file arrays.rst.

Having said that, I do think that the entire Arrays chapter could be usefully refactored into two chapters - one dealing solely with low-level implementation issues, and the other primarily dealing with linear algebra. It's quite a long chapter now...

[ViralBShah]

Yes, good idea to separate the array operations from linear algebra. Stuff like sparse matrices would then also get split into two. The other special matrices are largely linear algebra focussed, but one can do some basic non-linalg operations on those too.