v2.1

This version is compatible with Cyclus v1.6.

Fix 'provide more than its SWU capacity' FlexibleEnrichment bug

The reason why this bug appeared is unclear, it has never happened before.
The cause of it is probably some floating point imprecision, which can be
resolved/prevented by using Cyclus' AlmostEq function.

An example where the old version failed can be found in this GitHub Gist:
https://gist.github.com/maxschalz/ad3dd60dc44541f93c9d0f95a0a06f4d

Add FlexibleSink

Add FlexibleStorage

v1.0: Enrichment facility with multiple feed commodities

Please note that this facility is *not* backwards compatible because the
input file requirements are different.

v0.2: FlexibleEnrichment with one feed commodity

FlexibleSource present and FlexibleEnrichment only allows to use one
feed commodity.

v0.1: FlexibleSource