Update instrument definition of IMAT

[FedeMPouzols]

Fixes #11068. This adds the 4 monitors that we currently have.
This also adds IMAT in the ISIS facility in the Facilities.xml file.
For now the only technique given in Facilities.xml is 'Neutron Imaging'. More will be added later on.

To test:

• Verify that the changes to the instrument definition make sense. IMAT scientists are happy with the 4 monitor coordinates used, which are theoretical for now.
• A simple manual test can be done with a couple of commands like this:

ws=Load('D:/IMAT_data/IMAT00000000.raw')
Integration(InputWorkspace=ws, OutputWorkspace='imat_integ')

(you can get the IMAT00000000.raw file from the ISIS instrument data, cycle_15_1, this is just a test data file). The second line should integrate the spectra for the 4 monitors.

Note that this does not yet add the second camera as it was originally planned months ago in the issue description. We are not there yet, but the update included in this pull request is what we need for now.

As many changes are and will be happening around IMAT in the next weeks, I'm not including this in the release notes for now. We'll have to revisit the IMAT news as a whole when we have a clear picture of all the new IMAT stuff that will go in the next release.

[DanNixon]

What exactly is at the "centre" of IMAT (or at least what will be there)?

I was confused as to why the IDF positions for the monitors are so different to the sample-detector distance in Mantid until I realised the sample position was ~20m away from the origin.

[FedeMPouzols]

@DanNixon : the origin/center of IMAT is approximately the leftmost place in this schematic: http://www.isis.stfc.ac.uk/images/instruments/imat/imat-design-outline14029.jpg. The 4 monitors that are used at this stage (well in the end they splitted them into 8 temporarily) are located: one before the chopper, the second one after it, the third one closer or inside the cabin before neutrons arrive at the detectors, and the last one after the detectors.

Good that you pointed this out, cause it may well be that the zero was defined in an unconventional way because of no particular reason. I will double-check with the IMAT crew.

[DanNixon]

@FedeMPouzols Actually looking at the IDF it is obvious the "centre" is the source position.

It is probably fine as it is, I am just used to looking at spectrometers where the centre/origin is the sample position.