Merge branch 'develop-toymodel' into 'master'

Toymodel

A synthetic forecast generator for s2dverification which allows to test sensitivities with regard to predictability, forecast error, ensemble size and hindcast length. The toymodel allow furtherto test new functions in s2dverification and can be used for tutorial purposes. @fmassonnet and @eexarchou , could you review this function? Many thanks!

See merge request !35