Tabulating user fields of records is unimplemented

[jpswinski]

When a lua script tabulates a record, all fields in the records are copied into a lua table, except for fields that are user typed. These fields do not get populated.

What should happen is that the code should recurse down and create a sub-table.

https://github.com/ICESat2-SlideRule/sliderule/blob/3a029ff087dbf716b9f8236bae1c6f7b5420e512/packages/core/LuaLibraryMsg.cpp#L709

[jpswinski]

Also, need to support variable length binary fields. For instance, the H5File record has a variable length field called "data" that could be converted to a binary string and returned to Lua. The string.unpack could be used to decode the data if desired.

[jpswinski]

This also applies to the rec:getvalue function in the LuaLibraryMsg.cpp module.