Separate `poliastro.core` from the rest

In today’s community call, @jorgepiloto and I discussed the possibility to separate poliastro.core from the rest. This is somewhat related to #168, but uses a different method: by recognizing that poliastro.core is interesting and useful on its own, we split it off as a separate project, and make poliastro depend on it.

Luckily, poliastro.core only depends on NumPy and numba:

$ grep import src/poliastro/core/ -rh --exclude=*.pyc | grep -v '>' | sort | uniq
from ..angles import (
from .angles import E_to_nu, F_to_nu
from astropy import units as u
from ..elements import coe2rv, rv2coe
from .farnocchia import farnocchia, farnocchia_coe
from math import gamma
from numba import njit as jit
from numba import njit as jit, prange
from numpy import cos, cross, sin, sqrt
from numpy import cos, sin
from numpy import cross
from numpy import cross, pi
from numpy.linalg import norm
from poliastro.core.elements import coe_rotation_matrix, rv2coe, rv_pqw
from poliastro.core.hyper import hyp2f1b
from poliastro.core.stumpff import c2, c3
from poliastro.core.util import circular_velocity
from ..stumpff import c2, c3
from .util import rotation_matrix
import numpy as np

Advantages:

• Easier to contribute “raw algorithms” (see also our recent efforts in #1189)
• Users that just want the core functionality can just use the new project
• Users that prefer alternatives to astropy.units can bring their own (for example https://pypi.org/project/Pint/ and possibly others)

All of this is possible by installing poliastro and just ignoring the safe API, but the disadvantage is that it requires a lot of heavy dependencies (SciPy, Astropy, Plotly, matplotlib, others).

Disadvantages:

• More overhead
• We need coordination between the two projects

Thoughts?

[s-m-e]

Just some homework and an overview of all the places where poliastro currently, as of d85c9bd1e2e8c19765d34e6ee3d672cb51da22cc, talks to its core.

https://github.com/pleiszenburg/bulk_propagate/blob/3d132ace90cdb28252620fc2b5eee5cfed42cb20/core_interface_01.ipynb

[astrojuanlu]

We’re not doing this before 1.0.