minor updates in bioreactor

1. IdealFedbatch: made Sfeed a necessary parameter in __init__()
2. Added comments for __init__() methods
wrote production_envelope codes
removed class FluxEnvelope
renamed MdFBA to dFBAm
fixed bug in FVA.  added test_analysis.py
flux_envelope() and production_envelope()

added:
flux_envelope() and production_envelope()
to: variability.py

removed envelope.py