Adds a label in the table of the interval classification UIs
IMproves the content of the palettes' JComboBox.
We now have both range and discrete palettes. Range are proposed first
for interval classifications, discrete are proposed first for unique
value classification.
Improves rendering in the interval classification tables.
Changes the representation of double values in interval calssifications.
I'm playing with significant digits here... For values greater than one,
I don't want more than 4 numbers in the fraction part of the number. For
values lower than one, I want to be able to display things with at least
for significant numbers...

I use a trick here to be able to do that. For numbers whose absolute
value is lower than 1, I compute an approximation that keeps exactly
four digits and I display it normally. When editing it, the complete
value comes back. We loose accuracy for representation in the table but
the does not change things for the accuracy of the symbol
configuration...

In order to validate user input and to be able to parse inputs more
accurately, I use a JFormattedTextField for the edition.
We want to keep the value if it is left unchanged :-)
Merge pull request #457 from agueganno/interval-improvements

Add interval labels and color palettes in classifications

- Intervals are displayed in the editor of interval classification.
- We now have discrete palettes too. They are proposed first for unique value classifications while range palettes are proposed first for interval classifications.