Uncertainties in XPS traces

[vetschn]

Determining the uncertainty of the counts per second signal in xps traces from the phispe parser should be done in a better way.

I set it to be a constant, hard-coded value of 12.5 as the counts per second only seem to be taking values in steps of 12.5cps in the .spe files I've inspected (should investigate this further).

This counts per second signal should somehow be Poisson distributed, i.e. the uncertainty should probably be something like sqrt(n)​ but I know too little about XPS to really tell.