Converting hard-coded constants to enumerations

[alvinng4]

Currently, in both C and Python, we have hard-coded constants that look like this:

#define INTEGRATOR_EULER 1
#define INTEGRATOR_EULER_CROMER 2
#define INTEGRATOR_RK4 3
#define INTEGRATOR_LEAPFROG 4
#define INTEGRATOR_RKF45 5
#define INTEGRATOR_DOPRI 6
#define INTEGRATOR_DVERK 7
#define INTEGRATOR_RKF78 8
#define INTEGRATOR_IAS15 9
#define INTEGRATOR_WHFAST 10

It is probably better to convert them into enums.