Add Coordinate Reference System EPSG codes as attributes of Cpt object

[tlukkezen]

The pygef objects currently have coordinates (e.g. x, y and z) but have no proper universal Coordinate Reference System (CRS) definition. Only the "height_system" is a vertical CRS attribute, but the codes are linked to the GEF format, which is not a universal standard.

As a user I would like to be able to access an attribute of a Cpt object with the EPSG codes for both the horizontal (x & y) and vertical (z) oriented coordinates. These could be two attributes, e.g. xy_epsg, z_epsg.

EPSG codes are universally recognized geodetic definitions and have a scope way beyond geotechnical engineering, which makes a spatial object defined with epsg codes easy to work with by anyone.

The few CRS codes that are defined in the GEF format can be mapped to an EPSG code upon parsing, which will make the "height_system" attribute obsolete. See for instance the EPSG of RD and NAP (most commonly used in the Netherlands)