add XLALComputePSDfromSFTs and test script

Description

The function XLALComputePSDandNormSFTPower() introduced in !1294 (merged) is quite a monster with 11 inputs and 3 outputs. When extracting it from lalapps_ComputePSD I had already thought about disentangling it, but that would require more significant rewriting and probably some duplication, since (at least currently) the computation of the two quantities is just too closely linked. After discussion with @rodrigo.tenorio here is a new wrapper function XLALComputePSDfromSFTs that returns only the simple finalPSD vector and hides away the input options that are irrelevant to getting this most simple output. This should actually be enough for most users.

I'm also adding a test script for both the original and new function, though that's the WIP part, as it should still be extended to test a few more things.

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

@rodrigo.tenorio for a first look and then @karl-wette for formal review once I have finished up the test script.