Add GEO plot ratio

In the PR I add the possibility to create a ratio between two input file. The ratio is plotted as a GEO map.

Here an example:

• first file:test_data.pdf

- second file:test_data_additional.pdf - ratio first/second: ratio.pdf

The result can be obtained running:

bin/HexPlot --example 27

Moreover, in the PR labels have been enlarged and the color palette of the GEO plot was made finer.