With this PR transformations' parameters can be expressed in following ways:

• run-time Proj4 string

  srs::proj4("+proj=tmerc +ellps=WGS84 +units=m");
  

• run-time

  using namespace srs::dpar;
  parameters<>(proj_tmerc)(ellps_wgs84)(units_m)
  

• compile-time

  using namespace srs::spar;
  parameters<proj_tmerc, ellps_wgs84, units_m>
  

This PR also changes the way of processing parameters in general. In the original code some parameters like datum or ellps were expanded which resulted in addition of new parameters into parameter list. Default parameters could also be added. I removed this mechanism because otherwise I'd be forced to implement rewriting of static parameters in compile-time which would increase the compilation time. the old behavior is emulated though. Now instead of expanding parameters and adding defaults at the beginning, the parameter is searched on the parameters list and if it's not found only then it may be extracted from other parameter (e.g. ellipsoid from datum) and after that the default may be used. A side effect of this change is that the initialization with Proj4 string is 2x faster in my test which is creation of transformation:

from = "+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs"
to = "+proj=aea +lat_1=34 +lat_2=40.5 +lat_0=0 +lon_0=-120 +x_0=0 +y_0=-4000000 +ellps=clrk66 +units=m +no_defs"

Replacing string-based parameters with run-time parameters speeds up the creation another 5x. So WRT the previous implementation (string-based always modifying parameters by expanding them and adding defaults) the speedup is 10x.

Replacing run-time parameters with compile-time parameters speeds up the creation another 2x, so WRT the original 20x.

Furthermore it's still possible to initialize transformations passing SRID codes (EPSG, ESRI, IAU2000), but now newly introduced, non-string-based parameters are used for this which means the initialization is faster. Before Proj4 strings were used for this.

I also improved const-corectness of the code, e.g. the input coordinates of projections are not modified so projections does not rely on the internal caller to work properly. This was affecting combined projections (projection calling another projection).