Clarify notation and units in Ensemble Kalman Inversion documentation?

[glwagner]

A few clarifications, definitions of symbols, and perhaps units in the docs on Ensemble Kalman Inversion might improve the docs-reading experience. The docs now read

For example, the meaning of "curly N"

it's parameters, or the units of the covariance matrix are not specified. This leads to questions about whether the parameter update formula has the correct dimensions. The inner produce <., .> is also not defined.

[glwagner]

Written while reading the docs with @adelinehillier and @navidcy .