BUG: special/specfun: fix bug in mathieu_a for m=2 and small q (use only the power series there)

Adds a relative discontinuity |d| < 1e-12

qc = 2e-3
d = mathieu_a(2, qc-1e-16)/mathieu_a(2, qc+1e-16) - 1
TST: special: more tests for mathieu functions in test_basic.py
BUG: special/specfun: do not write out of bounds in Mathieu functions

Return NAN instead of writing out of bounds in FCOEF, MTU0, MTU12
BUG: special/specfun: implement expansion of Mathieu coefficients for q->0

The recursion used breaks down for small values of q, so it makes sense
to switch to a series expansion. The next terms are of order q^2 <
1e-14.