ANISE Python interface as a (partial) replacement for spiceypy

[ChristopherRabotin]

Hi folks,

I'm about to start working on #136 . I have a decent amount of experience with PyO3, so I hope it won't take me more than a few days to implement (my hopeful ETA is the new year).

I'm thinking that it should be used in a similar way to how the Rust interface works, so something like this (based on this test):

from anise import Almanac
from anise.time import * # This is a re-export of hifitime's Python library

ctx = Almanac().load("data/de440.bsp") # The Almanac() call creates an empty context

equivalent_ctx = Almanac("data/de440.bsp") # The parameter would be optional and allow loading _one_ file (variable args aren't support in PyO3 to my knowledge)

from anise.constants.frames import EARTH_ITRF93, EARTH_J2000
from anise.astro import Orbit

# Start by grabbing a copy of the frame.
eme2k = almanac.frame_from_uid(EARTH_J2000)
# Define an epoch
epoch = Epoch("2021-10-29 12:34:56 TDB")

orig_state = Orbit.from_keplerian(
        8191.93, 1e-6, 12.85, 306.614, 314.19, 99.8877, epoch, eme2k,
)

# Transform that into another frame.
state_itrf93 = almanac
        .transform_to(orig_state, EARTH_ITRF93, Aberration::None)

Let me know your thoughts before I implement it.