Add algorithm to extract time dependency of analyser polarised efficiency algorithm

[rbauststfc]

Add a new algorithm to find the polarised efficiency of the 3He analyser for a given time based on the fit results. This will make it possible to find the specific polarised efficiency at the time of any run during the experiment. This is more accurate than using a value that isn't time-dependent because the polarised efficiency of the analyser reduces over time.

With the extracted polarisation of the 3He gas (wavelength-independent quantity) interpolate the time dependence. The data analysis will also work assuming constant polarisation between periods.

Dirk has provided this information about what is required, in addition to a further comment below:

The code finds the time stamp of the measurement relative to a starting time. The transmission and polarisation of the 3He gas is then calculated for that time stamp based on the parameters established earlier in the calibration for the 3He cell (opacity, polarisation, transmission through cell,start time, decay constant).

This is a nice to have for the Polarised SANS reduction at ISIS.

Requirements still need to be gathered.