Electronic Boltzmann made compatible with master

[gitansh95]

1. lib paths changed while importing [examples]
2. calculate_EM_fields() code commented out in main.py [examples]
3. moment_defs are now in the form of functions. Utility file integrate_over_v.py being used [src]
4. charge (replacing charge_electron) and mass arrays of equal lengths added to domain.py [examples - mass to be removed from lib]
5. Added variable hybrid_model_enabled and set to False in params.py [examples - to be removed from lib]
6. Converted p-space start/end points to lists [examples]
7. Copied A_p() into a new C_p function in advection_terms.py [src]
8. riemann_solver replaced by riemann_solver_in_q and riemann_solver_in_p in params.py [examples]
9. fields_enabled variable added to params.py [examples]
10. mu and density changed to 4-D arrays, require change of indices [examples]
11. cell_centered_EM_fields() code commented out in main.py [examples]
12. A check somewhere prevents importing libraries into moment_defs.py. Manually values of pi and h_bar used for now [src - fix this]
13. Added function dump_aux_arrays() to lib/nonlinear/file_io/dump.py and ancillary code in nonlinear.py [lib]
14. Added t (time elapsed) as first argument to functions in advection_terms.py and as the second argument to RTA() in collision_operators.py[src]
15. instantaneous_collisions set to False in params.py [example - remove from lib]
16. Set source_enabled to True in params.py [example]