Merge pull request #2972 from argriffing/bench-onenormest

ENH: sparse: expm speedup for small matrices, through benchmarked one-norm estimation vs. calculation tradeoffs.

In the current scipy expm implementation, estimation of the 1-norm of
matrix products and powers dominates the computation time for the expm
of small matrices. While for large or sparse matrices this 1-norm can be
estimated using clever tricks that are much faster than exact calculation,
for small matrices it is faster to just multiply them out and compute the
1-norm exactly. This PR does some related benchmarking and adds an
arbitrary threshold below which the 1-norm is computed exactly within
the expm calculation.
expokit benchmark
TST: test expokit using the burkardt expm test suite
MAINT: remove non ascii character from the source code
MAINT: make expokit a private module, expose as method= in expm
CLN: sparse: clean up code in _expokit.expm a bit