Slightly clearer error message for incorrect ensemble sizes

[glwagner]

Perhaps instead of

EnsembleKalmanProcesses.jl/src/EnsembleKalmanInversion.jl

Lines 60 to 62 in da4b7da

	if !(size(g)[2] == ekp.N_ens)
	throw(DimensionMismatch("ensemble size in EnsembleKalmanProcess and g do not match, try transposing g or check ensemble size"))
	end

something like

"N_ens in EnsembleKalmanProcess and g do not match..."

or "Ensemble size N_ens in EnsembleKalmanProcess..."

The first time I saw this error, it took me a little while to figure out that N_ens is the ensemble size.