Add a JSON representation for the experimental design endpoint

[arteymix]

This can be achieved by implementing a new ExperimentalDesignWriter that writes JSON instead of a tabular output.

[oganm]

I am unsure if this is particularly necessary since samples endpoint provides all the relevant information

[arteymix]

The sample endpoint is confusing to use because it depends on a particular BioAssayDimension you're looking at and it does not put the experimental factors first in the hierarchy.