ENH: expm_2009 patches an expm 2005 weakness
TST: expm_2009 triangular test, and more accurate testing comparison for the overscaling example
DOC: expm_2009 docstring with a reference
MAINT: this conditional import is already unconditionally imported, so I am removing it; also this is a test commit
MAINT: expm_2009 replaced inefficient exact norm calculations with efficient norm estimates
MAINT: remove the inv import which is never used and is also clobbered
TST: linear operator testing
TST: linear operator transpose testing