Add EPSG "TARGET_UOM_CODE", "FACTOR_B", "FACTOR_C" fields to the uint_of_measure table of proj.db

[hlarsgaard]

In IOGP's P1/11 format geodetic transformation parameters are stored. Want to capture this information from proj.db
since PROJ is used to do transformations.
HC,1,1,0 is the Px/11 record defining the unit of measure. This record has a structure that mentions the base UOM of a UOM that is not the base unit. The EPSG database has a UnitofMeasure table that also has a reference to the base UOM. That field is TARGET_UOM_CODE.
In the proj.db database, the base UOM is hidden. It has a conversion factor, but to which UOM is not explicitly defined.
Add _AUTH_NAME and _CODE for the base of this UOM to the uint_of_measure table of proj.db

The EPSG UnitofMeasure table has two fields for conversion from/to the base UOM. It will be great if these fields can be kept in proj.db. They should be reflected in the Px/11 formats. Understand that it is inconvenient to have two fields instead of one for efficiency in calculations. It may be possible to add the two fields and keep the current conv_factor field.

[rouault]

we don't really need that in PROJ. What would be it useful for ?

[hlarsgaard]

IOGP provides the EPSG database. IOGP also defines formats for delivery of data. These formats need data from the EPSG database. Most of the required data is already duplicated in the proj.db. When we use PROJ to do transformations, it is a pity that we also need to fetch some minor parts of the required data from the EPSG database directly. It is just two records that need extensions to fulfil the requirements.