Make SleepScoreMaster robust to data with different sampling frequencies

[dlevenstein]

-Update all the code to load the LFP with bz_GetLFP, and to use the buzcode-style lfp structure throughout. This has the benefit that the sampling frequency is associated with the lfp structure (in lfp.samplingRate), and so nothing has to be hard coded but can use whatever sf the lfp is.
-Update bz_GetLFP to have an optional input ‘targetSamplingRate’.