Accounting for fixedFields in ExpGenerator

When specifying 'fixedFields' in experiment.json, we should not add encoders for fields not in this list. Before, expGenerator just looked at 'includedFields', which may not be the same as 'fixedFields' because a time field in includedFields gets expanded to three encoders. 

In the future, 'fixedFields' should be renamed 'fixedEncoders' and there should be a check that 'includedFields' and 'fixedFields' refer to the same fields.