Please refer to runtime_options() for a detailed description of
these options.
Note: Default options are set up when loading rkeops. To reset
rkeops options to default, you should use the function
set_rkeops_options(). To set up a particular option, you should
use the function set_rkeops_option().
Some wrappers are available to enable some compilation options,
see use_cpu(), use_gpu().