This commit converts the PSIBackgroundSubtraction algorithm to a data processing algorithm. This increases the clarity and transparency of the workspace history.

In order to do this, the operator overload which removed the background has been replaced with a CreateWorkspace call, which generates a workspace based on the backgrounds obtained, followed by a call to the child algorithm "Minus".

This first commit implements the fitting method to determine the background, this consists of the following steps:
(1) Define a flat background and ExpDecayMuon function
(2) For each spectra fit this function to the second half of the data
(3)  Subtract the flat background from the raw counts

This commit also adds a unit test to the Algorithm