[FEATURE] Conversion of measurement units

[tommens]

Describe the new functionality you would like to see

1. Convert values from a given measurement unit to another one. For example, we would like to convert some length measurement between (centi/deci/...)meters/feet/inches/miles/nautical miles/yards. Similar conversions would be for measurement of areas, currency, power, energy, pressure, speed, temperature, volume, weight and masses.
2. Any conversion has a "From" and a "To" field, it should be possible to either switch both fields easily, or fill in either one of those fields, in order to do conversions in both directions (for example, convert from Celsius to Fahrenheit and back), depending on which of the fields is filled in.

Illustration
As an example, you can take inspiration from how unit conversion is realised in https://web2.0calc.com
The figure below provides an illustration to convert energy from joules to kilowatt hours.