added implementation infrastructure to add preference values to exchange nodes
removed all message and market model usage. transformed the transaction class into an easy-to-use wrapper around the recording of a transaction
removed an inline that shouldn't have been there in the first place
added a test for getting a transaction from a trade and resource
updated cyclus.h and added missing headers in cmakelists
added documentation to the transaction class
added some missing header files in cmakelists
consolidated transacted resource table into transactions table
removed last reminiants of market models
removed any notion of transactions
updated cyclus.h
made some resource buff functions member-wise const
implemented sorting arcs in the greedy solver. needs tests
added test for request pref -> arc preference access for a node
added tests for greedy matching with preference sorting
added a docstring line about the preference ordering
removed all remaining mentions of markets
removed extraneous files from cyclus.h
added preconditioning infrastructure
preconditioner impl finished