Clarify intensity units

[k-wolfinger]

No description provided.

[bruhwiler]

I'm not sure how important this is, but we want to specify with SRW every time to use 'arbitrary' units.

As long as we are consistent with how fields are initialized, we should get results with units that we understand.

I'm not sure what units we decided not to use for intensity -- presumably Watts, Watts/m^2 etc.

[bruhwiler]

Lines 854 and 855 in pulse.py have some magic numbers for getting the intensity correct:
constant_factor = 1.198945869831954 # 7.3948753166511745
length_factor = constant_factor / (self.sig_s / params.nslice) # / self.ds

This code should be reviewed.