Add max field 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
I've created a PR adding discreteMax to Statistics in kvs-coder/HealthKitReporter#28.
An additional change here is needed to add a new field to Statistics class.

Describe alternatives you've considered
Not applicable.

Additional context
None.

[karolinaoparczyk]

I've created a PR with the change: #77