Fixed bugs relatedt to single trigger or empty files. Optimized ReadMVSCTriggers() function.
SFTutils: new API functions for resizing SFTs

- XLALMultiSFTVectorResizeBand(), XLALSFTVectorResizeBand() and XLALSFTResizeBand()
- can either expand (and zero-pad) or 'trim' the frequency band of SFT (vectors)
- refs #1464
ComputeFstat: trim 'running-median' SFT sidebands after use

- running-median SFT sidebands can dominate the SFT frequency-band for
  moderate ~O(1e3-1e4) Fstat bin searches, which has a large impact on
  the performance of resampling-Fstat: the SFT bandwidth input to
  ComputeFstat_Resamp() directly determines the number of samples to be FFT'ed.
- this patch fixes this bottleneck by 'trimming' the SFTs back to
  their minimal required bandwidth after the running-median
  noise-floor estimation is completed.
- refs #535, #1464