#151 getLongitude() wrong after setAxis()

[Jean-Marie Epitalon]

Hello,

I change lon and lat axis of a variable using method
setAxis (index, axis).
An internal attribute of that variable: _grid_ is not
updated.
As a result, a subsequent call of method
getLongitude() returns the longitude axis of _grid_
instead of the actual one.

When setAxis is invoqued, we should check if changed
axis is belonging to current _grid_. If so, just cancel
the grid by resetting attribute _grid_.

Jean-Marie