Make set_fields_from_vocs set bounds when required.

[shuds13]

If the generator is a libEnsemble generator, then when using vocs option to GenSpecs we may also use it to set lb/ub bounds, which is usually a user_specs.

gen_specs = GenSpecs(
    generator=gen,
    batch_size=batch_size,
    vocs=vocs,
)

we may want to set user field lb/ub from vocs variables bounds.