New module to perform decimal floating point arithmetic for charts.

This change adds a small module to perform floating point arithmetic
with a decimal base, and uses it to calculate the graticule marks for
charts.

Rationale:  Graticule marks want to be displayed in decimal.  However
not all decimal values can be represented in a double precision
floating point binary.  This approach pushes the precision loss from
the value of the mark, to the position on the chart - we don't
particularly care if a tick mark is a fraction of a pixel out of place,
but we do care if 4.0 is displayed as 3.9999999999

Reviewed-by: Ben Pfaff