Revert "read in constraints with weights, warning outputs"

This reverts commit 271e5820369dd46b4ffd6fd0dc1f65f8bb93862d.
geneval: wrote read_constraints(), some docstring edits
megatableau: re-wrote read_megt_file(), read_weights() to be more efficient. read_weights() now can cope with limited mixing of format types.
phlearn: minor edits so that functions will work as intended.