Add discreteMax quantity to Statistics

[karolinaoparczyk]

Is your feature request related to a problem? Please describe.
My project requires getting max, alongside average, in Statistics when getting infromation about heart rate.

Describe the solution you'd like
This could be easily resolved by adding a list of discrete quantities, instead of a single one.

Describe alternatives you've considered
Not applicable.

Additional context
I'm using https://github.com/kvs-coder/health_kit_reporter in my flutter project and will follow up there to facilitate getting the max value as well.

[karolinaoparczyk]

I've created a PR with the change: #28