enable optional use of resampling algorithm

As reported by Pep Covas, we currently seem to have the demod F-Stat algorithm hardcoded. My first suggestion was to just add a new "method" argument to the "ComputeFstat(BaseSearchClass)" class and change the line

FstatOAs.FstatMethod = lalpulsar.FstatOptionalArgsDefaults.FstatMethod

in core.py accordingly.

Pep tested something like this and got:

XLAL Error - XLALComputeFstatResamp (ComputeFstat_Resamp.c:470): Resampling does not currently support atoms per detector
XLAL Error - XLALComputeFstatResamp (ComputeFstat_Resamp.c:470): Invalid argument
XLAL Error - XLALComputeFstat (ComputeFstat.c:828): Check failed: (input->method_funcs.compute_func) ( *Fstats, common, input->method_data ) == XLAL_SUCCESS
XLAL Error - XLALComputeFstat (ComputeFstat.c:828): Internal function call failed: Invalid argument

So the most difficult part would be to look into how the semicoherent search could be adapted to not use the atoms flag.