lalapps_splitSFTs: use standard naming convention for narrowband SFTs

Description

lalapps_splitSFTs currently writes out SFTs with a very basic name, omitting even the .sft prefix. This patch implements a narrowband SFT naming convention conformant with the official convention for broadband SFTs. The format is described in the documentation of lalapps_splitSFTs:

  The name of the output SFT follows the standard SFT filename convention, with
  information specific to narrow-band SFTs included in the description:
    <outdir>/<obs>-<nSFT>_<det>_<timebase>SFT_NB_F<firstbin>_W<binwidth>-<start>-<span>.sft
  where
    <outdir> = output directory given by the '-o' option, default is '.'
    <nSFT> = number of SFTs in the file
    <obs> = 1-character observatory prefix, e.g. 'H', 'L', 'V'
    <det> = 2-character detector prefix, e.g. 'H1', 'L1', 'V1'
    <timebase> = time base of the SFTs, rounded to the nearest second
    <firstbin> = first bin of the SFTs, expressed in the form
      <firstbin> = <freq>Hz<remainder>, where
        <freq> = <firstbin> / <timebase>, rounded down to nearest Hz
        <remainder> = <firstbin> - <freq> * <timebase>, number of remaining bins
    <binwidth> = bin width of the SFTs, expressed in the form
      <binwidth> = <freq>Hz<remainder>, where
        <freq> = <binwidth> / <timebase>, rounded down to nearest Hz
        <remainder> = <binwidth> - <freq> * <timebase>, number of remaining bins
    <start> = start time of the first SFT as a GPS timestamp
    <span> = time spanned by the SFT in seconds
  If an output file already exists, the program will append the new SFTs to them, making
  it possible to construct the final narrow-band SFTs by running the program multiple
  times with different input SFTs. The GPS timestamps of the input SFTs need to be in
  ascending order to get valid merged SFT files.

An example narrowband SFT filename is H-1_H1_1800SFT_NB_F0299Hz180_W0010Hz1-12345678980-1800.sft. The format of the first bin and bin widths is meant to be both human readable (i.e. the SFT starts at ~299 Hz and is ~10 Hz wide) and machine readable if required (i.e. the SFT starts at bin 299*1800+180 and contains 10*1800+1 bins).

Closes https://git.ligo.org/CW/software/lalsuite/issues/69

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

Review Status

@david-keitel to review