Expose some internal options as configuration options

[david-zwicker]

We currently support some special options for differential operators, including conservative versions for spherical grids and extra checks for vectorial operators. These options are currently controlled via additional arguments to the factory functions, which is difficult to expose to the user in general settings. It might be much better to expose the settings globally via the configuration mechanism. This approach is generalizable for other options, but it has the slight disadvantage that we loose fine-grained control, e.g., it will then be impossible to choose different settings for different use-cases of the operators – one way to circumvent this (and to also keep backward compatibility) would be to change the default value of the options to None, indicating that they should be read from the configuration.

[david-zwicker]

Similar ideas would also allow us to support different stencils, e.g., a 9-point stencil for 2d Cartesian coordinates, which would then only be enabled in this very specific case.