Add support for multiple units

[dominikznidar]

We saw that in Amman they prefer to input blood pressure in cm[Hg] instead of mm[Hg] that is more common here.

Goal of this issues is to:

• find a way to accept data input in different units
• convert data before it's submitted to the server
• find a way not to loose precision during data conversion
• determine if we should store the original input unit

Units to also in this issue:

• weight (kg, pounds)
• temperature (celsius, fahrenheit)
• blood pressure (cm[Hg], mm[Hg])