now takes areas differences between edges following circles of constant latitude and edges following great circles into account during barycenter computation

* some reordering of operations
* improved documentation

* rbf can also be added to an interpolation stack configuration via the NNN routine

* thanks to Andrea for the hint

* in dark mode contents of latex generated formulas would be basically invisible

* this should espectially work well on vector machines
* it is enabled by compiling with "-DYAC_NEC_EXPERIMENTAL"

* routine yac_get_cell_bounding_circle uses specialised routines for most common cell type
* should improve performance

* originally this routine also computed the very expensive sine of the angle...