Feature/save vtk files

On the development branch, It's now possible to save scalar and vector field as (vtk, but also other) files using meshio. In the package namespace, the function write_field_to_file is available which can be used as

tetrax.write_field_to_file(field, sample)

or with the optional keywords

tetrax.write_field_to_file(field, sample, fname, qname)

As an alternative, the method AbstractSample.field_to_file(field, fname, qname) can be used, where the sample parameters is obviously omitted.

In the same vain, the resulting magnetization is now saved to the experiment directory after running ExperimentalSetup.relax(..., save_mag=True, fname="mag.vtk").