ENH: special._testutils: add option for accepting nans by default in tests
ENH: special/_testutils: make FuncData accept a comparison functions rather than only taking numerical data
TST: special: add systematic tests against mpmath

This systematically cross-checks values from (nearly) all special
functions that exist both in mpmath and scipy.
ENH: special/_testutils: add an option for ignoring the sign of infinity
TST: special: work around limiting value issues in lpmv & hyp2f1 tests
Merge pull request #500 from pv/special-mpmath-syst-submit

TST: special: test against (nearly) everything in mpmath