Typo in constant derivative transition model process covariance formula

[KaidosUncle]

The constant nth derivative linear transition model has a typo in the covar method on line 150. There is an N to the power of 2 when it should be N times 2 (extra asterisk).

Current: covar[l, k] = (igrand[l, k]*dt / (1 + N**2 - l - k))

Corrected: covar[l, k] = (igrand[l, k]dt / (1 + N2 - l - k))

[sdhiscocks]

Closed with #758. Thanks @KaidosUncle