Generic material properties

[paolafer]

This PR drops the optical part of the name of the OpticalMaterialProperties files and relative namespace. Since we use more properties that are not just optical (such as scintillation and TPB emission spectra, yield, attachment...) this seems more logical.
Two more non-optical properties, namely the Fano factor and the ionization energy for gaseous xenon, are added to the property table instead of being coded in the IonizationClustering class.
This PR closes #12.

[gonzaponte]

I've edited your comment to close the related issue automatically:

closes issue #12 -> closes #12

[gonzaponte]

These values correspond to
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.46.1676
which are GXe measurements.
For LXe, Doke measured Wi=15.6 eV in https://journals.aps.org/pra/abstract/10.1103/PhysRevA.12.1771
The same paper gives fano factors of the order of 0.05, which seem too low, but in
https://www.sciencedirect.com/science/article/abs/pii/0168900294012369
they also measure 0.033.

[paolafer]

I've changed the ionization energy. However, I'm puzzled about the Fano factor ( in NEXT papers we used the value 20 sometimes...) : maybe we should review also the value we use in xenon, then.

[paolafer]

See, for instance
PhysRevB.68.054201.pdf, where they talk about discrepancies between calculations and measurements.

[gonzaponte]

in NEXT papers we used the value 20 sometimes...

In gas? The discrepancy seems to be only in liquid. For gas I thought we always used 0.15, although the first paper above measured 0.29.

PhysRevB.68.054201.pdf, where they talk about discrepancies between calculations and measurements.

Not sure what to make of this... Should we use F=20 then?

[paolafer]

in NEXT papers we used the value 20 sometimes...

In gas? The discrepancy seems to be only in liquid. For gas I thought we always used 0.15, although the first paper above measured 0.29.

I was referring to liquid. Yes, we used 0.15, but it's probably time to review it, also.

PhysRevB.68.054201.pdf, where they talk about discrepancies between calculations and measurements.

Not sure what to make of this... Should we use F=20 then?

[gonzaponte]

I believe simulations in LXe need to go in a different direction because we need to introduce the anticorrelation between ionization and scintillation. This requires adapting nexus to this case, which may not be trivial. Also, we might want to use NEST (by the way, they use F=0.03 for LXe).