data with "nan" in coordinates can't be plotted ("nan" in data-values is fine!)

[raphaelquast]

currently, if a dataset with "nan" in the coordinates is attempted to be plotted, the kernel is interrupted! ("nan" in the data with valid coordinates is fine)

TODO

• check if all coordinates are valid
• mask for invalid coordinates prior to plotting (or at least issue a warning)

check

• check what part of the code can't deal with nan's as coordinates and how to deal with this efficiently
• avoid that this becomes a performance-issue for very large datasets!