[WIP] Theory covariance matrix plots in validphys2

Created by: RosalynLP

Hello, as discussed in the phone conference just now Cameron and I have added some features to results.py and plots.py in validphys2 to reproduce the plots in "RP+CV notes" in the wiki.

There are a few changes we're planning to make in the near future:

• Implement calculation of experiment and theory correlation matrices by taking in the corresponding covariance matrices as an input. Currently these are redefined within the correlation matrix action which is inefficient.

• Remove unnecessary instances of t0 checks. These appear in the definitions of theory covariance matrices in an analogy with the pre-existing experiment covariance matrix action, but we think it shouldn't be necessary. However, we don't understand exactly what this bit of the code is doing.

• Introduce user-controlled (as in inputted from runcard) limits for the colour bars in the matrix heat plots, and a legend which displays these values. Currently these limits are set values in the code.

• Add experiment and data set labels to the matrix plots so it's clearer which elements correspond to which data sets

• In the current output there seems to be a bug relative to some point in the past, where the theory covariance matrix has some very high valued elements:

This means the version in this pull request has output with some discrepancy compared the plots in the wiki. We are working on fixing this currently.