Set min/max values according to the min/max found in all time steps

[hugohartmann]

Describe the bug
Related to issue #64: When a field has constant values for a certain time step, and a graph is made on this time step, the resulting figure is completely black. When sliding through the time steps, the figure remains black.

Expected behavior
Set the min-max range according to the min/max found in all time steps of the simulation.

[dmey]

This is useful but we need to be careful as when the range computed from all time steps is very large, we may get the same results as we currently do. This is because the although the range in the time dimension is large, the range in the spatial direction (neighbouring pixels) is much smaller.

For example see this where the temperature at 2 m has a range of 30 over all time steps but the variability of the temperature between grid cells is 1 or 2 order of magnitude less. E.g. with the current grey scale colour map

I think that here the choice of colour map is critical and, even then, I was not able to get any good results with the colour maps currently available in QGIS when the range is so large.

@hugohartmann did you have a specific colour map in mind or a way to handle this case? The implementation should be general and work for diverse ranges like this one.