Use PostNewtonian.jl for initial orbital parameters

[nilsvu]

Proposed changes

Replaces the dependence on SpEC to compute low-eccentricity initial orbital parameters with the sxs package. It provides the PN approximations at higher PN order than SpEC, is much faster, and avoids spurious output from old Fortran code (LSODA) that was used in SpEC.
The PN approximations are provided through the PostNewtonian.jl Julia package, so Julia will be downloaded on first use, which may take a few minutes.

See also moble/PostNewtonian.jl#41, which made this possible.

Upgrade instructions

To use low-eccentricity initial orbital parameters for BBH initial data, install the sxs package in your Python environment:

# In the build directory:
./bin/python-spectre -m pip install sxs

Now you can use ./bin/spectre bbh generate-id with the --eccentricity=0 option.

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments