Add support to specialized physical models which are not present in core.

[lmdiazangulo]

Sometimes it might be necessary to add specializations of materials which are application specific. In the case that it is also need to read them from the .json file, the readPhysicalModels method must call two functions:

• readCorePhysicalModels, which follows the existing path and is non-virtual.
• readSpecializedPhysicalModels, which is a virtual method which can be overloaded by a specialization of parser so it can also work to read the new materials which are not known by opensemba core.

Once the two groups of materials have been readed, they can be merged.