Orbital elements

[altuson]

In this PR, we add the option to return the orbital elements (eccentricity, inclination, perihelion distance) of the queried target.

Changes in this PR include:

• Added functionality to return orbital elements (eccentricity, inclination, perihelion distance) from Horizons. Instead of returning the elements as a function of time, it returns the average value during the queried time. This was to simplify the handling of time scales, see below.
• Removed verbose parameter. All columns get returned all the time.
• Updated existing tests with new features and added new test test_orbital_elements().
• Updated README with new features.
• Bumped minor version number: 0.6.0.

A note on time scales

Horizons.ephemerides() assumes the input times are in UTC, whereas Horizons.elements() assumes the input times are in TDB. The output timestamps from these two queries don't match and it would be incorrect to merge the two tables assuming the times were the same.

Seeing as the orbital elements do not change appreciably during the length of a TESS sector, they definitely do not change significantly in the few minutes offset between UTC and TDB. Therefore, we take the average of the orbital elements during the time window and return those instead.

[jorgemarpa]

Loks good to me, will merge and push new version to PyPI