Added Physical Interface Device (PID) usage descriptions.

Also
1. Allowed ranges of PAGEs and USAGEs to be specified in the
initialisation data. You can now specify:
PAGE xx-yy  ...for usage page definition
and
xx-yy       ...for usage definition
Where, xx and yy are printable hex values. Useful for blocks
of reserved usages.

2. Changed getUniqueName to allow a context to be specified.
This is to allow the same field name to appear in different
structures without having an instance number appended. If a
field name does appear more than once in a given structure,
then an instance number will be appended to make it unique.