TST: special/test_mpmath: fix tests for j0 and j1 --- the results are numerically correct after all
TST: special/test_mpmath: put more weight on testing 1e0..1e5 parameter values
BUG: special: improve behavior of kn and kv at large orders and arguments

Use AMOS also for kn (the Cephes code has bugs for large orders), and
work around AMOS returning nan instead of 0 in some cases where we know
the function underflows.

Closes Trac #851 and #1589
BUG: special: improve accuracy of negative half-integer order complex-valued Bessel functions
BUG: special: fix sign of infinity in Bessel Y
BUG: special: initialize variables to NAN

In some cases AMOS doesn't actually compute results for IERR=3, so it's
best to initialize with NAN.
BUG: special: improve Bessel Y accuracy by using AMOS also for real values

The Cephes implementation is a fallback for cases when AMOS doesn't
return results.
TST: special: adjust bessel mpmath test parameters
BUG: special: fix bug in cephes/yv for large arguments
TST: special/test_mpmath: adjust spherical harmonics test tolerances
TST: special: disable hyp2f0 test for now
TST: special: fix passing-around of ignore_inf_sign
BUG: special: fix sign of infinity at jv(n, 0) for non-integer n < 0
BUG: special: resolve large-argument issues in Bessel J by always using AMOS

Closes Trac #1740
TST: special/test_mpmath: don't check sign of infinity for Bessel J

The inf sign from mpmath isn't always reliable either.
GEN: special: regenerate files
TST: special: enable two fixed knownfailures
ENH: unstable beginning of expm_action implementation
ENH: matrix power operator
DOC: stats: fixed a couple typos in the docstring of chisquare