Incorrect RA values displayed for grid lines

[halevin]

If the grid is active and one zooms to the source, the RA values of the grid are not correctly displayed. The values are repeated for different grid lines:

[bmatthieu3]

This was a bug occuring when displaying meridian labels in sexagesimal (HMS). The angular displayed precision has to be adapted when the unit is the hour and not degrees (h = d / 15).