Add mathematical description of link between error-covariance, error-correlation and uncertainty

[shunt16]

From Zhav Loizeau in #1:

Maybe a short paragraph describing how one obtains the error-covariance matrix from the error-correlation matrix and the uncertainty vector would be good. Actually having such a note early on to dis-ambiguate, and be clear about what should be stored in memory?

[shunt16]

Agreed, I will implement this with @pdevis

[EmmaWoolliams]

And make really clear the distinction between correlation along a particular dimension that you've specified in your array, and correlation between different quantities stored in the same file

[EmmaWoolliams]

This is part of a broader issue. This specification assumes the reader understands NetCDF, arrays, uncertainties, error correlation structures and lots more ... it may not be possible to explain all that in here, but we have to point to "introduction" resources otherwise people will be quickly scared off