XLALComputePSDandNormSFTPower() normalizes input SFTs in place

@karl-wette as discussed in !1456 (merged), XLALNormalizeMultiSFTVect() and by extension the PSD-computing functions from !1294 (merged) and !1456 (merged) modify the input SFT vector in place, which for a user just wanting a PSD is unexpected. This wasn't a problem as long as there was only the monolithic lalapps_ComputePSD, but now that I made the SWIG-accessible function, it's inconvenient. On the other hand, computing PSDs over large data sets would become much more memory-intensive if a local copy were always made. So my suggestion would be:

1. Give XLALNormalizeMultiSFTVect() XLALComputePSDandNormSFTPower a new boolean argument normalizeSFTsInPlace which is false by default and makes a copy of the inputs internally unless set to true instead.
2. Have XLALComputePSDfromSFTs() call it as false, as it's intended to be an "as easy as possible, no surprises" user-friendly function.
3. Have lalapps_ComputePSD call it as true because in this case there won't be any need to reuse the inputs.

Does that sound reasonable? It'll require an API change though.

Edit 20201223: fixed function name in 1.