Substance statistics endpoint;

[matthiaskoenig]

Endpoint to query substance statistics: /statistics/substances/

{
    version: 0.9.2,
    substances: {
        caffeine: {
            studies: {
                count: 20, 
            },
            interventions: {
                count: 20,
            },
            outputs: {
                count: 20
            },
            timecourses: {
                count: 40
            },
            scatters: {
                count: 30
            }
        },
        ...
    }
}