Compensation + autofluorescence = truncations

[jakebeal]

Line 53 in readfcs_compensated_au is:
if(floor && numel(CM.Channels)>1), no_AF_data(no_AF_data<1) = 1; end
By default, we use the floor, and if there is a significant level of autofluorescence, this ends up making big spikes on the left hand side like this:

It is not clear whether this can be readily handled with a smarter application of compensation, but we need to investigate and either fix or document.