lalapps_splitSFTs: drop -i argument, support concatenated SFTs

Description

3 patches to lalapps_splitSFTs and associated tests:

1. Drop support for the -i argument for input files; since these always have to been last on the command line, they're really positional arguments, so don't need an optional arguments (although -- is supported as a separator.)
2. Add support for concatenated SFTs. Code now loops over all SFTs in all input files, rather than just reading the first.
3. Whitespace/indentation/style cleanup, delayed from 2nd patch so that those changes are easier to see. Also do style cleanup on splitSFTs.c for easier legibility.

Closes https://git.ligo.org/CW/software/lalsuite/issues/77

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

Removing -i argument is backwards incompatible, but this code is not widely used in CW (yet) so impact should be minimal.

Review Status

@david-keitel to review.