Riemann wrapper mapping vector to pointwise solver

[rjleveque]

riemann/rptest/1d/rp1_driver.f90 contains a wrapper for the vectorized solver to give a pointwise solver taking a single pair of states. Intended for testing solvers and for use in notebooks. See the Makefile to figure out how to run or use with f2py.

Improvements needed:

[ ] Add aux_l, aux_r, and problem_data as input parameters, so calling sequence is the same as pyclaw solvers such as riemann.acoustics_1D_py.acoustics_1D

[ ] Modify riemann/rptest/1d/rp1_driver_test.ipynb to test new version.

[ ] Extend to riemann/rptest/2d/rp2_driver.f90 for normal solver based on an rpn2 subroutine.

[ ] Extend to transverse solver.

[ ] Add travis tests.