Read depletion_chain information from main cross section file
Created by: drewejohnson
This PR closes #1239 by replacing the environment variable
OPENMC_DEPLETE_CHAIN in favor of a
depletion_chain node in the main
OPENMC_CROSS_SECTION file. Unless a
chain_file argument is directly passed when constructing an
openmc.deplete.Operator object, the chain is created using this new default file. A
DeprecationWarning is raised alerting the user to the change in behavior.
A unit test was added that makes a dummy
cross_sections.xml file with this node, swaps the
OPENMC_CROSS_SECTION environment variable to use this file, and creates a
Operator. The depletion chain on this operator is that from
chain_simple.xml, and is compared to a
Chain created directly from this file.