MAINT: minor performance improvement

Merge two loops and move invariant of switch case out of inner loop.
Improves performance by 5% - 10% depending on the order.
MAINT: tell compiler that edge interpolation is less likely

This should be true for most cases where performance is relevant.
Improves performance by about 5% on images > (500, 500) with order three.
MAINT: add map_coordinate tests with non-continous arrays
ENH: stats: Some enhancements to the chisquare functions:

* Add the `axis` argument to stats.chisquare and stats.mstats.chisquare.
* Add the `ddof` argument to stats.mstats.chisquare.

Closes gh-2108 (trac #1583).
TST: special: fix test_e1_complex & test_ei_complex

Don't try to compare in regions where function changes too fast compared
to float precision.
BUG: special: fix infinite runtime in hyp2f1
BUG: special: avoid loss of precision in gegenbauer
BUG: special: fix negative order chebyshev polynomials
TST: special: enable mpmath orthopoly tests
BUG: special: fix negative order jacobi polynomials
TST: special: enable jacobi test
TST: special: fix laguerre polynomial test
BUG: special: fix negative order legendre polynomials
TST: special: fix gegenbauer + legendre tests
BUG: special: fix loss of precision in eval_gegenbauer (int-order)

Use power series instead of recurrence for small x.
BUG: special: fix loss of precision in eval_legendre (int-order)
TST: special/mpmath: adjust parameter ranges probed
TST: special/mpmath: check the small-x expansion regime for gegenbauer and legendre (int-order)
TST: special/mpmath: adjust bei/ber ranges
TST: special/mpmath: add explanations for the redefined functions