Fix crash when n_init < sim_workers

[AngelFP]

Addresses #133.

Problem:

• Optimas was crashing if n_init (the number of Sobol trials for initialization) is smaller that the number of simulation workers. This is because Ax cannot generate the n_init + 1 trial until at least one of the Sobol trials has finished, and therefore raises an exception.

Solution:

• Ensure that at least 1 trial is completed before moving from the Sobol to the BO step, and keep generating Sobol trials until that happens, even if the number of Sobol trials exceeds n_init.

[delaossa]

Looks good. Thanks Angel!