minor fix in MBGravitationalWaveTransient for low maximum frequency

MBGravitationalWaveTransient is broken when maximum frequency << sampling frequency / 2, since fddata and full_inv_psds have components up to the maximum frequency and they are shorter than ifo.frequency_mask for such a case, which causes errors like IndexError: boolean index did not match indexed array along dimension 0; dimension is 4097 but corresponding boolean dimension is 8193. This MR fixes it, as well as adding unit test for such a case. cc: @gregory.ashton @pawan.gupta, who reported this bug.

@colm.talbot I just let you know this bug-fix MR, in case it can be included in the next release.

Closes #711 (closed)