only enable valid variables for plotting

in this commit, we use the plotmethods check_data method to only select the valid variables for plotting

closes https://github.com/psyplot/HZG-Internals/issues/1

• https://github.com/psyplot/psyplot/pull/22 merged
• https://github.com/psyplot/psy-maps/pull/10 merged
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)