Allow non-zero starting omegas in XRC

Currently we assume that we start do the grid scans at omega = 0 then omega = 90. This is fine for testing but we should make sure that the code is generic enough that we can do a grid scan from any starting omega.

## Acceptance Criteria
* The motor positions put in the nexus file take into account the value of omega
* Omega is also correctly put into the nexus file
* We remove the check at the start of the grid scan for omega to be at 0