- proj_obj_create_projected_XXXXX() are renamed to
  proj_obj_create_conversion_snake_case() and just instanciate
  a Conversion object
- Advanced manipulation functions are moved to a dedicated
  section at bottom of proj.h
- New C API needed for GDAL OGRSpatialReference

- createFromPROJString(): take into account axisswap step for Krovak and Transverse Mercator (South Orientated)
- Geocentric export to PROJ4: use datum when possible, and add explicit units=m
- ESRI WKT parser: make it case insensitive to parameter and projection names, and more tolerant about possible parameter name aliases
- import from WKT1 for Polar_Stereographic: don't be case sensitive
- importFromPROJString: allow pm to override datum
- Equidistant cylindrical: add support for non-standard latitude of natural origin, used in a GDAL test case
- tmerc export to PROJString: use 'k' instead of 'k_0'
- pj_ellps: use official value from EPSG for reverse flattening of Airy ellipsoid
- GDAL compatibility: add support for importing odd formulations of Mercator as WKT1, but rejecting them when exporting to PROJ
- Add export of 'Geostationary Satellite (Sweep X)' to WKT1_GDAL via EXTENSION.PROJ4 node
- importFromPROJString: add support for +f
- WKT1 / PROJ4: add support for EXTENSION.PROJ4 nodes and +wktext
- exportToWKT: change way we deal with AXIS by default for WKT1_GDAL
- Improve etmerc handling
- Fix WKT import of peg_point_heading for Spherical_Cross_Track_Height
- International Map of the World Polyconic: change parameter mapping
- exportToPROJ: add alpha parameter
- Hotine_Oblique_Mercator_Two_Point_Natural_Origin: GDAL_WKT1 related fix
- GDAL compatibility improvements in import from PROJ4 / WKT1 for polar stereographic
- Add support for +towgs84 when importing a +proj=geocent
- import from WKT1: add support for an odd Mercator_1SP formulation handled by GDAL
- export to proj4 strings: add +units=m to projected CRS for better GDAL compatibility
- export to proj4 strings: add +no_defs to CRS for better GDAL compatibility

This work mostly consists of:
- a C++ implementation of the ISO-19111:2018 / OGC Topic 2
  "Referencing by coordinates" classes to represent Datums,
  Coordinate systems, CRSs (Coordinate Reference Systems) and
  Coordinate Operations.
- methods to convert between this C++ modeling and WKT1, WKT2
  and PROJ string representations of those objects
- management and query of a SQLite3 database of CRS and Coordinate Operation definition
- a C API binding part of those capabilities

This is all-in-one squashed commit of the work of
https://github.com/OSGeo/proj.4/pull/1040