ligolw_inj_snr: Implement SNR rescaling

- Two types of SNR rescaling have been added:
    - Normal: Rescale injection SNR according to a Gaussian distribution with a given mean and standard deviation
    - Uniform: Rescale injection SNR to a uniform interval
- Random seed is set by the MD5 hash of the provided xml document

NOTE: This will fail, currently, for more than a single detector, since it rescales the single detector optimal SNR, not the network SNR. This is because the rescaling happens for an intrinsic parameter (hrss, amplitude, egw) for a *sim_burst* not for any given detector, so rescaling with multiple detectors will rescale appropriately only for the last detector considered.
test_binj.sh: A few more examples