wakeflow errors and warnings should be changed to adhere to standard Python conventions.

Currently, wakeflow uses print statements to warn users about certain parameter choices, and generic Exceptions for incompatible or otherwise non-sensical parameter choices. This does not really adhere to standard practice; warnings.warn should be used for the warnings, and more specific errors should be used instead of Exceptions. For example ValueError for a bad parameter choice. The current implementation is due to my limited knowledge of error handling at the time I wrote the code. Users should be able to catch certain warnings and errors and handle them appropriately if they desire, which is impossible in the current implementation.