Optimizations to multiband weights

This MR makes a few optimizations on computing multiband weights in MBGravitationalWaveTransient.

• This MR speeds up weights computations by optimizing the computations of window function at evenly-spaced frequency points. For a 1.4Msun-1.4Msun BNS signal observed from 5Hz, this optimization reduces its run time from minutes to seconds.
• This MR adds save_weights and setup_multibanding_from_weights methods, which can be used to save/load multiband weights as done for ROQ. This can save the cost of precomputing weights for the second execution. Type checks are updated so that the likelihood can properly load numbers from a .hdf5 weights file. Unit tests are also added for testing this in/out.
• Add minimum_frequency, maximum_frequency, and number_of_bands properties as they are frequency used for setting up multiband weights.

I have verified that the multiband weights produced by the new codes are the same as those from the previous codes in this notebook.