Lowlevel bindigs to the GR plotting framework

Based on version 0.37.0, the one installed by hombrew on osx (brew install libgr) at the time of creating the bindings.

To install, first install libGR and set the GRDIR env variable, then run opam install gr.

The documentation is published here: online documentation.

Currently libGr is only looked via GRDIR or through the default library paths of your system. You can customise this by unsetting GRDIR and specifying the path to libGR.so (or libGR.dylib) with the environment variable LIBGRPATH.

The tests are only compiling the examples. You can run them as follows:

$ GRDIR=~/gr dune exec examples/example_simple.exe
$ GRDIR=~/gr dune exec examples/example_complex.exe
$ GRDIR=~/gr dune exec examples/example_lowlevel.exe
$ GRDIR=~/gr dune exec examples/example_dynamic.exe

Make sure that GRDIR or LIBGRPATH are pointing to the right path.