lalapps_SFTclean output improvements

Description

@karl-wette Here are two three more improvements to lalapps_SFTclean that would be great to get into the upcoming release.

1. implement same comment handling as in lalapps_splitSFTs - this is controlled by the same --addComment=[0,1,2] argument, with the new default 2 meaning a comment will be appended to the original.
2. add --outSingleSFT mode
   • SFTs will be written to a single file per IFO
   • now always using a MultiSFTCatalogView to loop over SFTs in per-IFO order
   • using canonical output file names from XLALOfficialSFTFilename
3. in LALRemoveKnownLinesInMultiSFTVector() use basename() of linesfile to determine detector prefix, allowing to pass absolute filename paths to the app

(But for now I've kept the original logic with single-SFT calls to LALRemoveKnownLinesInMultiSFTVector(); I'd need to study the internals more carefully to know if it's safe to change to a single call to that one instead.)

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

Test script has been extended to cover the --outSingleSFT mode.

As commented in the code, there are two assumptions I'm relying on:

1. XLALSFTdataFind() returning a catalogue with SFTs sorted by increasing GPS-epochs (always true as per documentation)
2. LALRemoveKnownLinesInMultiSFTVector() properly applies only those files with canonic IFO names matching the single-IFO inputSFTs (seems to be its whole point)