[Amor] Scale by proton current

[jokasimr]

There is a proton current log in the Nexus files produced by Amor. It should be used to normalize the counts both for the sample measurement and the reference measurement.

Specifically, assume the intensity in the detector is proportional to the integrated current in the proton beam over the time of the measurement:

I(\lambda, \theta) = R(Q) I_{referece}(\lambda, \theta)  C_{t_0, t_1}

where $C_{t_0, t1}$ is the proton current integrated over the time of the measurement and $I{referece}$ is the reference intensity per unit of integrated current.

[jokasimr]

Note this comment here: https://github.com/scipp/essreflectometry/pull/99#discussion_r1812255785

The tests will need to be updated when this is implemented.