GNUPlotter Macros

This crate contains the macros used to generate the plots using gnuplot. The macro's consider plots to be defined using structs that derive the Plot macro. This will both provided setter functionality for the plot as a builder would, but also provides validation functions regarding the plot's content. Different from a builder, a plot is not necessarily produced / visualized only once, and so it needs to build itself, so it can keep building by adding new data series (for example).