Merge branch 'feature/plotting_none' into 'master'

When previously wanting to specify no gridLines on a plot gridLines="" was
used this could be extended to the use gridLines=None as well which is more
consistent with usage for specifying boolean variables.

See merge request [dune-fem/dune-fempy!62]

  [dune-fem/dune-fempy!62]: gitlab.dune-project.org/dune-fem/dune-fempy/merge_requests/62