multiple evaluation of a formula (as when computing function curves with
many points or doing fits with user-defined functions) more efficient.
For example, loading the v0.2.1 test file from bug #2736453 is around 4-5
times faster now.
Also, the new code is supposed to be better documented, more readable,
adhering to the new coding style as described in the LabPlot Wiki, and
generally more suitable for being ported to the new code base.