Create example notebook for Gaussian covariance

[ntessore]

Create an example notebook that runs a simple Gaussian simulations with correlated maps for positions and shears, then analyses these external maps with Heracles, producing spectra and covariances.

This will demonstrate

• how to use Heracles with external maps (Utility function for reading external maps #286) and
• how to use the covariance code in a well-known setting (full sky Gaussian).

Apart from the benefit as an example, this notebook can also serve as validation for the jackknife, comparing to the known full-sky Gaussian covariance.

[JaimeRZP]

#290