OMCI-lib-go: Code upgrade

This JIRA is to cover the following updates to the OMCI-LIB-GO library:

• The main message decode/serialize file (messagetypes.go) and related unit test file has been broken down into 16 separate files oriented on OMCI message type functionality. This allows unit tests to report code coverage on message types more clearly.  No API changes or OPENONU-go or BBSIM code changes required.
• Code generation covers more of the ME's that were not previously decoded & code-generated (only 35 remaining out of a total of 288 MEs defined by the standard). See *** Note below.
• Remaining Extended message set message-types are now fully supported.  No API changes or OPENONU-go or BBSIM code changes required.
• 40 bytes baseline messages now supported (no OMCI trailer required). Helps to clean out a vendor specific check in omci_cc.go (line ~289) of the OpenONU adapter if desired.
• Uses the latest G.988 standard: ITU-T G.988 Amendment 3  (3/2020)
• Incremented the major version number

PDF slide presentation related to this issue: VOLTHA OMCI-LIB-GO update.pdf

*** The main impact to existing code with the new code generation is the Multicast Operations Profile ME.  Was previously hand coded. Now fully code-generate so names of some attributes and the ME itself changed slightly.