Fstat: exportable XLALComputeFstatFromFaFb as wrapper to internal compute_fstat_from_fa_fb

Closes CW/lalsuite#34

I've replaced all occurrences of XLALComputeFstatFromFaFb() in lalpulsar/src with the internal compute_fstat_from_fa_fb(), so the newly exported XLALComputeFstatFromFaFb() wrapper is not actually called from anywhere within lalsuite, but it allows to do e.g. in python:

import lalpulsar
lalpulsar.ComputeFstatFromFaFb(0,0,0,0,0,0,0)
4.0

Passes make check for me; @karl-wette does this look safe?

PS: I keep forgetting the right cross-project linking syntax, will later update the commit message with an absolute URL.