Adding category to physical properties

[JosePizarro3]

I think it will be useful to start adding some more definitions to PhysicalProperty (until Area D fixes this, I will simply add them as quantities).

One of them which might be useful is categories. This should be a MEnum with shape to define properties belonging to a certain category (electronic, magnetic, optical, thermodynamic, etc.). This might help us later when normalizing and identifying one or another type of property for searches, and it is information which can be extracted from the ttl file from the taxonomy task force.

@JFRudzinski what do you think? @markus1978 I think this is relevant also for you, it just means that the PhysicalProperty(Section) definition class you are doing will now have another attr called categories.

[JFRudzinski]

@ndaelman is this relevant to your Area D discussions?