[ANALYZER-1817] and [ANALYZER-1818], related to GEO map visualization

[ANALYZER-1817] Geo Map visualization needs to handle null/missing/NaN/NA Measure values consistently when used as size by and color by values
[ANALYZER-1818] Geo Map visualization needs to handle negative Measure values consistently when used as size by and color by values
[FIX](pentaho.DataTable/DataView#getColumnRange) null values could cause the result to return a null minimum and a non-null maximum.
[FEAT](pentaho.DataTable#getColumnRange ABS) Added argument options, supporting option 'key', that can receive a function that extracts the value from each DataTable's original value. This is used to obtain a range for the absolute value of the size by measure.
[FEAT](pentaho.VizController.getDarkerFromColorHex) Added new method that serves to darken a color given as an array of RGB components. Based on protovis' pv.Color#darker method.
[CHG](ccc_wrapper Scatter HeatGrid) Slightly increased the dots' line width, so that the GeoMap and the CCC visualizations look the same.