rpmodel v1.2.3: update for CRAN compatibility

Handling unintended usage; ci:ca for C4

Implemented tests and continuous integration. Corrected bugs related …

…to vectorization.

bugfix for vectorization

Set negative values of the temperature-dependent kphio to zero

corrected man page of rpmodel, description of kphio

Bugfix for vectorization

v1.0.6 implemented provisional (uncalibrated) C4 option

set negative vpd to zero

Added public functions and Jmax