Make use of numba opt-in not opt-out

[ricardoV94]

Calling forest_plot when numba is installed ends up with cryptic errors: ValueError: Failed in object mode pipeline (step: remove phis nodes) This is a PITA.

Please consider making the use of numba opt-in (not simply by having numba installed).

[tomicapretto]

If we wanted to do this, I think (almost?) everything is already there. We just need to make sure that

https://github.com/arviz-devs/arviz/blob/7c1637f6d4574bc8ba5757165f9e7bb02e202621/arviz/utils.py#L200

is not True by default when numba is installed (which is the case now)

[OriolAbril]

Sounds good to me, the only change would be in https://github.com/arviz-devs/arviz/blob/7c1637f6d4574bc8ba5757165f9e7bb02e202621/arviz/utils.py#L328 make it False instead of the result of numba_check.

I'll also look into the error, it might not change much between using numba there or not if its in object mode.

[drbenvincent]

+1: I'm hitting this error as well