Addition of new classes to represent the extended concept of digit.

We include two pairs of classes:
1. the 'digitopodial skeleton' ie proximo-distal series of phalanges plus a metapodial
2. 1+soft tissue
Note the 2nd concept typically does not correspond to a distinct subdivision of the
whole hand or foot (e.g. in humans this is continuous in the metapodial region)
Addresses issue #420