Consistently refer to pre/post-conditions in docstrings

[vyasr]

This is a very minor issue, but we are currently very inconsistent with our usage of pre-condition vs pre condition. However, precondition (without a space or hyphen) is an accepted word, as is postcondition, so we should replace these consistently in the docs.

[bdice]

The docstrings side of this issue is fixed by the combination of #387, #389, and #392. However, internal names like _collect_pre_conditions and _OPERATION_PRE_CONDITIONS still exist. Those should be replaced with names like _collect_preconditions and _OPERATION_PRECONDITIONS. I think these changes would be confined to flow/project.py and tests/test_project.py.