Add support for placeholder in kernels paths

Currently, the kernels argument in TourConfig support a kernel path string input or a list of kernels path strings, with explicit relative or absolute paths:

TourConfig(
    kernels=[
      'lsk/naif0012.tls',
      'pck/pck00010.tpc',
      ...
    ],
)

Like for the metakernel, it should be possible to support placeholder in the path string, eg. '$KERNELS/lsk/naif0012.tls'. This value should be override by the kernels_dir argument:

TourConfig(
    kernels=[
      '$KERNELS/lsk/naif0012.tls',
      '$KERNELS/pck/pck00010.tpc',
      ...
    ],
    kernels_dir='/data/JUICE/kernels',
)

Then, the kernels should be loaded as /data/JUICE/kernels/lsk/naif0012.tls and /data/JUICE/kernels/pck/pck00010.tpc.

It should also use the spacecraft argument and the KERNELS_{SPACECRAFT_NAME} environment variable (like in a .env file) if kernels_dir=None (default).

Going one step further

By default the $KERNELS placeholder is the most frequent in metakernel files but any PATH_SYMBOLS value could exists. Replacing any placeholder in a metakernel is already possible with the Metakernel object (see docs) but it is not possible directly in kernels_dir (only the $KERNELS placeholder is replaced at the moment).

It should be possible to provide a custom dict with eventually, multiple placeholders, to replace custom PATH_SYMBOLS:

TourConfig(
    kernels=[
      '$JUICE/spk/xxxx.bsp',
      '$CLIPPER/spk/xxxx.bsp',
      ...
    ],
    kernels_dir={'JUICE': '/data/JUICE/kernels', 'CLIPPER': '/data/EUROPA_CLIPPER/kernels'},
)

If theses values are present, they should override the default environment variables in both the metakernel and the kernels list. In that case: kernels_dir='/data/JUICE/kernels' is equivalent of kernels_dir={'KERNELS': '/data/JUICE/kernels'}.

This could be extended to the .load_kernel() as well (tour and traj):

new_traj = traj.add_kernel('$CUSTOM/new.ck', custom='/data/my-simulations/')

It will be recommended to use only $UPPERCASE_PATH_SYMBOLS.

It should also be possible to provide the same kind of dict to the remote_kernels argument to point to different remote sources based on the placeholders names:

TourConfig(
    kernels=[
      '$JUICE/spk/xxxx.bsp',
      '$CLIPPER/spk/xxxx.bsp',
      ...
    ],
    remote_kernels={'JUICE': 'https://spiftp.esac.esa.int/...', 'CLIPPER': 'https://naif.jpl.nasa.gov/...'},
    download_kernels=True,
)

In that case: remote_kernels='https://spiftp.esac.esa.int' is equivalent of remote_kernels={'KERNELS': 'https://spiftp.esac.esa.int'}.