histogram based min max values. environment vals

histogram based min max values. environment vals

The y axis was inverted becase the way the fits files are saved. This
has been fixed. I now use DATAMIN DATAMAX environmental variables to
pass values to the loader. At the moment this only works if both are
defined but it might be useful to split this. If these variables don't
exist the we do histogram and base the min and max on this, very similar
to ds9's 99.5 scaling.