Create markdown representation

[dellaert]

This PR adds markdown formatting for discrete factors and conditionals. Main new functions are

• DiscreteConditional::_repr_markdown_
• DecisionTreeFactor::_repr_markdown_

These method names are not very c++ like but work in python notebooks as is. Waiting for a PR in wrap so we can rename them to "markdown", in analogy to "dot". @varunagrawal :-)

An example:

Key formatters are supported but need a bit more machinery on python side, e.g.,

class MD:
    """Small class that renders markdown with optional arguments."""
    def __init__(self, obj, *args):
        self.md = obj._repr_markdown_(*args)
    def _repr_markdown_(self):
        return self.md

[varunagrawal]

Should I only do markdown for now and add other methods later? Or is there some subset of functions that you would find immediately useful.

[dellaert]

Should I only do markdown for now and add other methods later? Or is there some subset of functions that you would find immediately useful.

It's just a loop over strings, right? Just add all supported formats, including mime.

[varunagrawal]

Straightforward PR.

[varunagrawal]

Minor comment: auto&& will automatically take care of constness.

[dellaert]

Cool, thanks for joint effort! I will merge after CI passes.

[varunagrawal]

Woohoo