Multi Value Measurements

[derpeter]

Use case

Some measurements e.g. Blood Pressure consist of multiple values. While this could be entered into wger via 2 custom measurement this feels wrong. Especially as the the date picker in web / app only allows to set a date not a time (not sure if i should open a second request on that) so the measurements cant be linked together by timestamp. E.g. https://codeberg.org/toz/MediLog medilog is a nice example for these multi value measurements tracked via an app.

Proposal

Extend the custom measurements to allow to define multiple values per measurement.
Optional: Add some of the common ones as selectable defaults. e.g. #875 lists already most common ones.