Internal helper for getMeanLength(). Inverts the length-weight relationship
\(w = a_i l^{b_i}\) of each species at every weight in params@w. The
result is a weighting factor for sizeIntegral(), so it is a point value on
the grid and is not bin-averaged here.
length_at_size(params)A species x size matrix of lengths in cm.
A MizerParams object.