Merge dynamic dimension values

[philippjfr]

Merges the values from multiple composed DynamicMaps addressing #920.

[jlstevens]

The code is clear enough, but it might be nice to add a doctest example here (if we are using doctests).

[jlstevens]

Looks like my comment in the issue regarding a KeyError was confused. Anyway, merging dimensions with merge_dimensions makes sense - my only question, is there anywhere else we should be using it?

[philippjfr]

my only question, is there anywhere else we should be using it?

I think this uses it everywhere it matters, i.e. when merging HoloMaps/DynamicMaps in an Overlay and when computing the dimensions during plotting to handle Layouts/GridSpaces of multiple HoloMaps/DynamicMaps

[jlstevens]

Thanks for adding the doctest!

Happy to merge once the tests pass.

[philippjfr]

Some syntax error due to the test being multi-line.

[philippjfr]

Ready to merge when it passes.

[jlstevens]

Tests have passed. Merging.