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
Linking existing digit concepts to extended concepts. Issue #420
replacing CSnn with HsapDvs for EHDAA2 local copy. Issue #434
 * limb/fin: digits issue #420
    * 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)
 * NTs for dorsal and ventral optic cup. #419
 * NT: skin scent gland. NT: interdigital gland
equiv axioms for digits. Issue #420.
equiv axioms for digits. Issue #420.