Adds warning if data rate < 95% in load()

Closes #74 (closed)

Code to test:

import toolbox_scs as tb

proposal = 5836
runNB =  302
from extra_data import by_index
run, data = tb.load(proposal, runNB, ['PP800_DelayLine', 'BAM1932S', 'FastADC5raw', 'SCS_SA3'])

run, data = tb.load(proposal, runNB, ['PP800_DelayLine', 'BAM1932S', 'FastADC5raw', 'SCS_SA3'],
                   subset=by_index[0::4])

One issue is that hRIXS users will get a lot of warnings since the camera operates at low rep rate. We could disable the warning if the mnemonic contains hRIXS in it, but soon we will change to Jungfrau at 10 Hz. What do you think @kuikenb?

proposal = 2953
runNB =  511

run, data = tb.load(proposal, runNB, ['hRIXS_det', 'hRIXS_delay'])