Remove assertions
Use min() to set the value of 'data_min'
Use max() to set the value of 'data_max'
Update comment
Fix AssertionError raised by PreferencesDialog.__init__()

The two assertions used to make sure than the minimum and maximum
values that the gtk.SpinButton's may take do not always hold, raising
the AssertionError (assert data_min <= vmin) exception. The reason for
this is the linear normalization formula used by APLpyNormalize, which
may set a value of 'vmin' smaller than that of the minimum pixel level
of the finding chart — or, analogously, a value of 'vmax' greater than
the maximum pixel level. Kudos, once again, to Scott Engle for letting
us know about this issue (#34).

This merges branch 'issue34'