Add new plotter to interface with uhepp.org

Create a new (plain python) plotter class that outputs histograms as JSON files and pushes them to a collection at uhepp.org.

To use this feature, the visualize config would need these additional lines:

plotFormats: json
plotter: uhepp
# Collection id, e.g. 1
plotter.uhepp.collection: 1

To interact with the web API, the user would need to setup the following environment variables:

• UHEPP_TOKEN with the API token
• UHEPP_API with the API endpoint (e.g. https://uhepp.org/api/)

The Python module would depend on

• json and
• requests

which should be available on most systems. If these packages are not available, the rest of CAF must not be affected by failing imports.