Fix bug when all classes are hidden (#55)

* Docstring and typing fixes

* Deprecations and private methods

* Fix bug when all classes are interactively hidden

Hiding all classes would throw an exception related to mismatched
data types during some internal update of the widget state. I traced
that back to the `SankeyPlot.plot.data[0].link` update and found it
broke if you used the link data from a `go.Sankey` that was
initialized with `link_kwargs`. Instead, intitializing with an empty
`go.Sankey` seems to solve the issue. That only arises if all classes
are hidden, so I just threw in an early return of an empty `go.Sankey`
in that case.

Switch to MIT license

Bump version: 0.2.2 → 0.2.3

Bump version: 0.2.1 → 0.2.2

Bump version: 0.2.0 → 0.2.1

Bump version: 0.1.0 → 0.2.0

Check for geemap import before throwing geemap error

Bump version: 0.0.6 → 0.0.7

Bump version: 0.0.5 → 0.0.6

Bump version: 0.0.4 → 0.0.5