ENH: begin the expm 2005 -> 2009 update; no tests yet
BUG: doing this commit because git told me to
TST: all tests pass, with default expm set to expm_2005
TST: tests pass, with expm_2009 as the default expm
MAINT: hide expm2, expm3
TST: do not import expm ecplicitly
MAINT: put back expm2 and expm3
TST: put expm back into scipy.linalg.matfuncs testing
MAINT: for logm(expm()) test only non-low precision
MAINT: sparse.linalg: remove expm_2005 in favor of the 2009 version
Merge pull request #2485 from argriffing/expm-2009-retry

ENH: scipy.sparse: update expm to Higham's 2009 algorithm