Create terms for number density

[dosumis]

The degree of concentration of countable objects (particles, molecules, phonons, cells, galaxies, etc.) in physical space. - https://en.wikipedia.org/wiki/Number_density

synonym: count per unit length, area or volume.

Subclasses:

• area density (move from under mass density and redefine - count per unit area)
• volume density (move from under mass density and redefine - count per unit volume)
• linear density (move from under mass density and redefine - count per unit length)

Background - in discussion on uPheno call 22/03/10 the many examples of area and volume density phenotypes we reviewed were count densities, not mass densities. see https://docs.google.com/document/d/1WrQanAMuccS-oaoAIb9yWQAd4Rvy3R3mU01v9wHbriM/edit#bookmark=id.cg486iwvs5b5

[sbello]

Related ticket #173

[dosumis]

Having looked at area, volume and linear density definitions, they are all consistent with being types of mass density. We should rename them all to 'x mass density' and create terms under 'count density' for area, volume and line count density.

We should also obsolete 'dense' - as this is just a weird and probably meaningless grouping term.