Fix up initial conditions

[langfzac]

• [x] Allow integers to be given as parameter values (ie. Elements(m=1)).
• [x] Change ϖ (\varpi) to ω (\omega) in code and fix terminology in docs.
• [x] #83
  • [x] Add additional orbital elements to Elements type.
  • [x] Rework kepler_init to handle e=0.

[langfzac]

@ericagol How should we proceed with the derivatives for the e=0 case? For now I just set anything that depends on ecosomega and esinomega 0, but that's not the correct way to handle the 1/e terms.

[ericagol]

@ericagol How should we proceed with the derivatives for the e=0 case? For now I just set anything that depends on ecosomega and esinomega 0, but that's not the correct way to handle the 1/e terms.

Good question @langfzac. I'll have to look into this.

[ericagol]

I think we decided not to address this right now, so the user will have to assume a small eccentricity for modeling circular orbits. If we do address this, a paper by András Pál (2009) could be helpful:

Pál (2009)

[langfzac]

@ericagol Ok, I just set the derivatives to zero for now, and made a note about it in the documentation. Unless you see any issues, I'm going to go ahead and merge.