Add 'set zero' and 'set ground' tools

Added two new tools: ```Set Zero``` and ```Set Ground```.

The ```Set Zero``` tool sets the 'zero' point for the following values:

* x, y and z (used in the 3D plots at the bottom)
* t, 2D distance and 3D distance (used as x axes)

The ```Set Ground``` tool sets the ground level for the elevation calculation.

Both tools are momentary. As soon as the user clicks, the tool returns to what was previously selected.