BUG: limit input ranges in special.nctdtr
BUG: return nan when ncp is too large for cdftnc
BUG: overflow in cumtnc.f

Fix an overflow bug in cumtnc.f. So the range of the nc parameter
is enlarged to [-1e6, 1e6]. However, a larger nc parameter may still
result in total loss of precision.
TEST: add test for the overflow bug in cumtnc.f
Merge pull request #3222 from richardtsai/nctdtr_inf_loop

BUG: special: limit input ranges in special.nctdtr

There seems to be no efficient algorithm to calculate the
PDF of the noncentral t distribution in high accuracy when
the noncentrality parameter is large. The limitations will help
avoid overflows and give a reasonable approximation under
practical conditions.

This also avoids the code getting stuck in an infinite loop.