Adding HTML holomap export to notebook archive

[jlstevens]

This WIP PR is attempting to address #449.

Unfortunately, it isn't working yet as bokeh JS isn't included in the output. I see various issues:

• I think __call__ should simply be the data saved to file by save.
• The __call__ method should not be used internally e.g in the html method.

By making __call__ closely related to save, users using __call__ will get useful, self-contained output which is currently not the case.

If we fix those issues, I'll have to think about whether the changes I've currently made to the renderers needs to stick or go away.

[philippjfr]

Closing, will try to fix this in #3836