Feature - continuous photon emission

Continuous photon emission: Moving call to random()<photon_sample_fraction so it's not always evaluated

Merge branch 'ChrisArran-continuousPhotonEmission' of https://github.com/ChrisArran/epoch into ChrisArran-continuousPhotonEmission

Merging main branch into local copy

Chris Arran - Extended photon emission tables

Continuous photon emission: Prevented particles from giving away more momentum than they started with.

Rewrote the expression as the Landau-Lifshitz radiation reaction force and checked if the momentum change is greater than the original momentum. If so, reduced momentum to zero and reduced the weight of the emitted photon to reflect the reduced photon momentum.

The synchrotron power was given in the limit beta->1, which causes problems and segmentation faults when a photon suddenly goes from being at rest to in a strong field. Added the correct factors of beta to resolve this.

The qed deck was being handled such that use_classical_emission and use_semiclassical_emission options could contradict each other and set classical emission but not continuous emission, which is not how it is currently intended. Added a warning and an or statement.

Photon downsampling: Increased the weight of the photons that are produced to keep total average photon numbers the same. Forced photon_sample_fraction<=1 to keep this consistent.

Photon downsampling: Added the same option to epoch2d and epoch3d, using the photon_sample_fraction keyword in the qed deck - this randomly discadds photon macroparticles at creation to reduce the amount of memory used.

Photon downsampling: In epoch1d, added the option to reduce the number of photons produced using the photon_sample_fraction keyword in the qed deck - this randomly discadds photon macroparticles at creation to reduce the amount of memory used.

ChrisArran-extendedPhotonEmissionTables: Updated the tables for photon emission to a) extend the range up to eta=100,000, well above where the model will break down; b) regrid the ksi_sokolov table ona more consistent, continuous 2D grid

Non-conforming tab character in epoch 2d version of photons.F90.

Added missing END IF and THEN. Fixed some misplaced tab characters while I was at it.

In the epoch2d and epoch3d versions I forgot to define the new variables in photons.F90/generate_photon. That should be fixed now.

Same process as before with the same keywords semiclassical_photon_emission and classical_photon_emission.

Same process as before with the same keywords semiclassical_photon_emission and classical_photon_emission.

These are controlled through the qed block with keywords semiclassical_photon_emission and classical_photon_emission with radiation reaction calculated from the average synchrotron power accordingly.

Mainly bug fixes, affecting photon initialisation and injection and the production of low energy photons.

Photon initialisation fix (issue #502)

Fixed bug with multiple subset maps (issue 361)

V4.19.2

Bug fix: photon energy extrapolation