Empty filelist into GPIData should throw a useful error

[Jason Wang]

dataset = GPI.GPIData(filelist, highpass=True)

when filelist = []

Should produce a better error than this:

data = data.reshape([dims[0] * dims[1], dims[2], dims[3]])

IndexError: tuple index out of range"

Something like, "you've passed in any empty filelist"

[Jason Wang]

Added exception throwing in 8e7e382