Ask the user before overwriting output layer with the same name

[vdeparday]

As it stands by default, overwrite is set to true so the calculation output overwrite the layer with the same title which could be the previous calculation or another layer that would happen to have the same name (e.g. see issue #4).

Shouldn't we be conservative and not overwrite by default?
Ideally, the user would be prompted to overwrite or not the existing data.