A specific unit shouldn't be attached to a variable label

[khider]

Describe the bug Currently, the units are added as a variable description. However, the same variable can have two different units (e.g. temperature in degC and degF) and two different variables could share the same unit (eg. Water flux and heat flux). So we should treat them separately.

[hvarg]

To solve this (without modifying the ontology) we are:

• showing name, unit, and description for each variable on variable selection.
• not allowing the user to create the same variable with the same unit.
• showing the message "Please prefer to create a new variable instead of editing the units of an existing one." on variable edition/creation.

If we want to have unique tuples (variable, unit) linked to the model, we will need to modify the ontology and add something similar to parameters on the model level.