New model_options statement

The purpose of this statement is to specify the options that apply to the whole
model, when there are several “model” blocks, so as to restore the symmetry
between those blocks (since otherwise one “model” block would typically bear
the options, while the other ones would typically have no option).

It still remains possible to specify the options on one “model” block, or even
spread accross several ones.

Closes: #19