Jitting or not jitting, that is the question

[marcocuturi]

Reflecting on #192

While I think it is more elegant to deprecate solvers in function form (e.g. sinkhorn.sinkhorn), I have some doubts on disabling automatic jitting. For many users not familiar with Jax and who might want to give OTT for the first time, not ensuring their code runs at the best speed might be a problem. Maybe add @jax.jit decorators on top of solvers. BTW, jit on jit remains usually optimal. This would also solve some of the OOM errors we have seen when not jitting code.