Conversion of Variable's Units of Measurement

Added the tounit( , ) function, which allows to convert variable's units of measurement.

The function has two arguments: a variable, whose value (with a set or not set unit) will be converted, and a target unit. The target unit can be defined either as a text string (eg "mm"), or as another variable, whose unit will be used.

If units of the first argument are compatible with the target unit (i.e. they have the same type), then the conversion to the target unit is performed. For example "kg" -> "t".

If units are not compatible, then the the target unit is assigned without conversion. Unit incompatibility error is not generated.