Conditional Formatting improvement ideas

[martinRenou]

Those are some improvements ideas following this PR: #10

• Add support for replacing a renderer, will be fixed by Handle renderer update #14
• Support for AND and OR operators for the Predicate class. The OR operator is already implicitly implemented (when providing a list of predicates).
• If there is a change in the renderer model, only the region/column affected by this renderer should be repainted We now let phosphor handle the repaint.
• We should compute Scales domain automatically, given the data. This could be an optional feature, first because it could be slow for huge data, second because in the case of streaming data we cannot really know if the domain will stay the same.

Advanced ideas:

• Implement a custom AST grammar for compiling renderers from Python functions?
• Use vega-expression for conditional formatting functions

[martinRenou]

As discussed with @wolfv, it might be smarter to directly use vega expressions for filtering and conditional formatting. It would provide a powerful and safe way of creating advanced filtering/formatting functions.

[martinRenou]

See #17 for the integration of Vega-expression + a Python to Vega-expression transpiler