library/GSM_RR_Types.ttcn: refactor IaRestOctHH coding

According to 3GPP TS 44.018, section 10.5.2.16, IA Rest Octets IE
starting with 'HH' bits may contain one of the following CSN.1
encoded components:

  7 6 5 4 3 2 1 0  Bit Numbers
  H H 0 0 . . . .  Packet Uplink Assignment
  H H 0 1 . . . .  Packet Downlink Assignment
  H H 1 . . . . .  Second Part Packet Assignment

We already have (partial) support for the first two, while the
last type has not been supported so far. Let's add it.

Also, this change introduces several templates for IA Rest Octets
IE and some of its components mentioned above. This would allow
us to abstract the API users from dealing with further changes,
e.g. adding a coding attribute 'CSN.1 L/H' and missing fields.

Change-Id: Ib3a21e7c5fa1cad4466e3a09fa70540de7f6ecc5
3 files changed