add conduit::utils::format

[cyrush]

Adds conduit::utils::format methods that support fmt style formatting with args encoded in conduit::Nodes.

While generally useful, the primary aim of these functions is to help us support for non-trivial domain maps (#476)

• add basic docs
• add python support

[coveralls]

Coverage decreased (-0.2%) to 86.226% when pulling 5fb35e5 on task/2021_01_utils_format_args_and_maps into 3539e80 on develop.

[cyrush]

@xjrc @agcapps was hoping for a quick review from someone else, but the update of the docs outputs includes a lot of noise. The most helpful thing would be to read the new docs and see if they make sense. I suspect the easiest way to do so will be after this PR is merged :-).