Refactor plotters

[oerc0122]

Description of work
Refactor all the plotters to use generator constructs, gridspecs and attempt to reduce code-duplication.

Fixes
Prevent number of plots corresponding to number of datasets.

To test
Try to break plotting with weird sequences of button presses.

[MBartkowiakSTFC]

I have been testing the plotter. What I found so far is:

1. Grid plots of 1D datasets have an empty legend.
2. If I plot into Grid or Single with legend switched off, and switch it on afterwards, I get an exception: <class 'AttributeError'> 'NoneType' object has no attribute 'set_visible'
3. Heatmap plot is labelling the plot axes wrong. In my test, time was shown as Q and Q was shown as time.

4. Heatmap plots start off without any legend. If I switch legend off and on, the legend appears.
5. If I try to plot a vector summary from the plot creator like this

the plot type ends up being Single and not Vectors:

While we do not want to allow Vectors plot type for normal data sets, it should still be possible to use Vectors for vectors.

6. For Single and Heatmap plots, switching the legend off and on makes all the subplots resize.
   Before:

After: