Update to include Manchester photon emitter code

[andrewgait]

Repeating https://github.com/UoMResearchIT/RTP-215-Photon-Propagation-Issue-Only/issues/33 for the new forked repo:

• Add code from previous Manchester fork to enable the photon emitter and (re)build from source
• Test new code against results from previous fork
• Test new code on HEP cluster
• Write tests for photon emitter (in src/emitters/tests) but also add something testable to the MPhys/Single_Emitter directory - to be tracked in Write tests for single_emitter #4.
• Rework branches to preserve history

[ianhinder]

Andy will tweak the branch structure so that his original work on top of yangocean/mitsuba3-manchester's main branch is just pushed into this new uomresearchit/mitsuba3-manchester repository. This should show all the original history linked up correctly. 🤞

[ianhinder]

The tests are being tracked in #4.

[andrewgait]

Branches in UoMResearchIT/mitsuba3-manchester now tweaked so that commit history in yangocean-sudo/mitsuba3-manchester is preserved (into the main and then dev branches of the new fork).