Flint: Fix default x axis

Closes #4 (closed)

Here is a fix to specify a default x-axis for some scans.

Flint do not guess this axis any more from the acquisition chain.

This change provides a way to make it work with old BLISS 1.8 (by providing a dummy add_curve_plot). This add_curve_plot method is available from BLISS 1.9 and also with the last BLISS 1.8.x.

There is maybe other fscans to fix such a way.

This was not tested.