resample data

[lucywowen]

• by default, resample signal to 64 Hz (or something else low-ish but reasonable…also should be a power of 2)
• allow user to specify outputted sample rate

function for resampling: https://docs.scipy.org/doc/scipy-0.19.1/reference/generated/scipy.signal.resample_poly.html

Internally call self.get_slice() to slice out a time range so that you first re-sample the data and then chunk the resampled data.

[lucywowen]

default is not 64Hz, instead if you don't pass a resample_rate, brain object stays the same.

this was also done in place