Testing sparse sketching : use theoretical floating point error bounds to define tolerances

[rileyjmurray]

This PR is a work in progress. I'll add a proper explanation once it's ready for review.

Need documentation for ...

• The new buffs_approx_equal function in RandBLAS/test_util.hh.
• sparseskop_to_dense in test_sparse.cc
• reference_lskges in test_sparse.cc