Redshift IC Check

This checks in the initial conditions to see if we have a redshift present. If we do, then we compare this against the redshift from the parameter file.

Redshift was chosen to prevent conflicts between the use of the Time varibale between SWIFT and Gadget-oid codes.

Let me know if this implementation is utter crap, or if there's anything that I need to change.

This change has been requested from users in the COLIBRE fork.