Remove `fpconst` in favor of `numpy`

Example in gstlal_inspiral, but in many other places throughout:

try:
	from fpconst import PosInf
except ImportError:
	# fpconst is not part of the standard library and might not be
	# available
	PosInf = float("+inf")

NaN, +/- inf can all be leveraged through numpy at this point.