Axion field integration

[jgalan]

A PR intended for the integration of axion-field equations into TRestAxionFieldPropagationProcess.

• Implement the use of TRestComplex inside TRestAxionField
• Implement basic axion physics validation pipeline, at pipeline/physics/AxionPhysicsValidation.C.
• Implement a method to integrate the field profile inside TRestAxionField.
• Implement the inhomogeneous field integration into the physics validation pipeline.
• Implement the field integration into TRestAxionFieldPropagationProcess
• Add a condition into TRestAxionFieldPropagationProcess to reject those photons not coming out from the end-bore. This task is now integrated into TRestAxionTransmissionProcess. See PR TRestAxionTransmissionProcess. Fixing the location of the window #38
• Added a macro loading validation to the pipeline.
• Added a dedicated validation pipeline for TRestAxionFieldPropagationProcess, at pipeline/ray-tracing/axion-field/.

List of other relevant changes.

• TRestAxionPhotonConversion has been renamed to TRestAxionField.
• TRestAxionField now uses directly the new TRestComplex object.
• TRestAxionBufferGas now implements proper REST-for-Physics units.
• TRestAxionEvent few un-necessary members have been removed.
• TRestAxionAnalysisProcess added new observables thetaAngle, phiAngle and R.
• TRestAxionAnalysisProcess has been documented.

The pipelines added at pipeline/physics/ and pipeline/ray-tracing/axion-field/ may serve as an example to use the routines implemented at the following PR.

The following plot can be generated as follows:

restManager --c photonConversion.rml
restManager --c plots.rml --f AxionPhotonProbability,root

These plots are the result of an arbitrary particle generator at the entrance of the magnet. It is not the corresponding to real solar axion flux.