Implement Plottable class

[tmancal74]

Use it in DFunction, CorrelationFunction, SpectralDensity and AbsSpectrum and other classes to control plotting.

[tmancal74]

It seems to me to make more sense to implement a Plotter object that would be able to plot the objects occurring in Quantarhei, rather than implement plotting methods in individual classes.

Advantages:

1. the classes would not contain a difficult-to-test code (which the plotting part always is). All Matplotlib code would be concentrated into one class or subpackage.
2. One can remove the dependency of Quantarhei on Matplotlib (one dependence less is good)
3. In principle, this could also be an independent package.

[tmancal74]

I leave the issue open for now, but soon I will create a new issue to get rid of the Matplotlib dependency. I will also create a new project (something like QrheiPlotter, or QPlotter)