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 sending the next value of *dt*.

In addition, this change:

 * Reduces code duplication between interpreter and codegen: the
   module *dagrt.events_python* is created to hold code for events
 * Improves the interpreter documentation
 * Makes a few stylistic fixes to the Python code generator

The version is bumped to 2019.5