Support user-facing adaptivity in Python + doc improvements

Closes #29

This adds a dt_next parameter to run and run_single_step to support adapting the time step. Additionally, the generator returned by run supports receiving the next value of dt via the send method.

In addition, this change:

• Reduces code duplication between interpreter and codegen: the module dagrt.events_python is created to hold code for events
• Renames StepCompleted.current_state to StepCompleted.current_phase
• Improves the interpreter documentation
• Adds a dt attribute to StepFailed
• Makes a few stylistic fixes to the Python code generator
• Addresses an interface inconsistency between the interpreter and the code generator: while the code generator raised a StepError in the case of a Raise, the interpreter raised whichever type was supplied by the Raise statement. This change modifies the behavior of Raise in the interpreter to also raise a StepError. The previous behavior required the argument to Raise to be a Python exception type, which went against the language-agnosticity of the representation.
• Updates the adaptive_rk example to use the new adaptivity interface.

The version is bumped to 2019.5

Leap test (should not require any changes): leap!31