Add Heading unit and type data to GeoPackage

[Didymograptus]

Proposal: Include a table 'UNTY' that holds the data used for types and units for each heading used in the file. The current version of the plugin only includes the UNIT and TYPE info from the respective groups.
Benefit: This will enable users to compare units used for headings across files and also standardise with the version created by the AGS Desktop Validator.

[RichMere]

...or use a field alias to display the field header, units and data type in each attribute / data table. e.g:

the LOCA_GL heading would become LOCA_GL (m, 2DP)

I think it's important the units are preserved somehow. Also in a future version it would allow the QGIS file to be saved as an output file which preserves the units and data types.