heev: document and check for square MPI grid and lower triangular matrix

[mgates3]

• heev function now asserts that the MPI process grid is square, and that the matrix is lower triangular. Note that slate_assert throws an exception and is not disabled by -DNDEBUG, whereas C++ assert exits and is disabled by -DNDEBUG.
• Documentation is updated with comments about both conditions.
• test_heev checks that heev throws the appropriate exceptions, rather than just skipping the tests.
• Also updated doxygen layout to the latest, since doxygen renamed "Modules" to "Topics", which we rename to "Routines".
• Also fixed a minor tester bug that caused a compiler warning (using pointer after deleting it).
• Use "2-stage", meaning 2-stage reduction to bi- or tri-diagonal as the more common terminology (cf. ELPA 2), instead of "3-stage", the 3rd stage being reduction to diagonal.

[mgates3]

Reviewed.

[hfp]

The restrictions for (P)HEEV(D) are:

0. Matrix extents need to be an exact multiple of the blocksize.
1. Lower triangular storage Hermitian matrix.
2. Square MPI process grid ($p \times p$).

What are the chances to relax the limitations?

I am currently hitting 1) with my application (via ScaLAPACK interface), but likely all limitations making Slate not applicable.

[mgates3]

I don't think (0.) is true. A SLATE matrix can be any size, unrelated to the block size.

Due to limited funding, the other limitations are unlikely to be addressed soon.