Work around int_max_str_digits limitation

This MR fixes FIRtools.py raises ValueError with recent bug-f... (#13) by temporarily increasing the str->int digit limit to support the pre-computation of factorials for the Bessel functions.

I'm not sure this should be relied upon as a long-term solution, but I don't understand the purpose of the code that is triggering the error in the first place. This MR is fundamentally to document the patch introduced for the gstlal-calibration-1.4.0 conda packages.