Add option to specify MLE fronal-length scale via file

[gustavo-marques]

The MLE dynamical length scale introduced via USE_BODNER23 improves the current approach of using a constant (1 km) length scale. However, USE_BODNER23 leads to excessive restratification at low latitudes, and the current implementation is a show-stopper for CESM3 fully coupled simulations. As a fallback, we would like to have the option to specify MLE frontal-length scale climatologies that can be constructed using the results reported in Bodner et al. (2023).