support custom projections (PROJ.4 parameter syntax)

[bastiK]

It should be possible to enter a custom projection using an ellipsoid (a=..., b=...), datum transformation (grid file or 7-parameter transformation) and one of the implemented base projections (Mercator, SwissObliqueMercator, TransverseMercator, LambertConformalConic). I would suggest PROJ.4 parameter syntax for the config string.

Libraries, I am aware of are Proj4J, Java Map Projection Library and GeoTools. The first two are derived from PROJ.4. Proj4J seems to be quite buggy and Java Map Projection Library is unfinished (e.g. no datum transformation).

Currently, my plan is to not use any libs, but code this manually and add to JOSM core. Then a plugin could simply provide more base projections and grid files.

[bastiK]

In 5066/josm:

Proj parameter refactoring (see #7495)