Consider threshold solar elevation in static projection method for consistency with the non-static one

Some better documentation, a new method to project a radiation from the horizontal to a slope (as needed by Snowpack), a foolproof method for computing the splitting (it only gets one parameter, the measured radiation, and knows which potential radiation to use...). Now Snowpack can use a Sun object for computing its radiation!

Since the autotools are obsolete (and unmaintained) in MeteoIO, they have been removed. The popc++ version can be compiled with cmake (with some tricks: disable GSN and force the compiler to popcc). 

For consistency, the various definitions of ot_deg and to_rad have been suppressed and replaced by Cst::to_deg or Cst::to_rad. Some mentions of the Earth's radius have been replaced by Cst::earth_R0. In Coords, some variables have been declared as const.

Outputs that were used for warnings (or information) are now done on cerr instead of cout, in order to avoid poluting the outputs of user code. Should we do the same for the exceptions?

A new method, getSolarTime has been added to SunTrajectory.

Several HACKS have been removed (either the HACK had been fixed but the comment was still there, or it has now been fixed, or after more (careful) consideration, it has been decided that the HACK was not a HACK)

A new group of classes has been created, under the meteolaws subdirectory. This contains various general meteorological laws, such as a sun radiation model, standard atmosphere, etc

The first implementation (not tested yet) of a non-linear least square fit algorithm has been implemented in the libfit1D. 

Several documentation issues have been fixed (obsolete code examples, etc) and classes grouped by modules. This should help the user to quickly locate the classes of interest.