Improve kernel management

Added

• New kernel-download/mk-download command line interfaces are available to simplify kernel downloads:

kernel-download --esa JUICE mk/juice_crema_5_0.tm

mk-download --kernel-dir /data/kernels/ mk/juice_crema_5_0.tm

More info here. Close #53 (closed).

• New .add_kernel() method on TourConfig and Trajectory to append new kernel(s) to new objects:

new_tour = tour.add_kernel('custom.ck')

new_traj = traj.add_kernel('new.tf', 'new.ti')

Supports for multiple kernels inputs is also implemented. Close #57. Example of this new method will be available in #56 to illustrate the use of switch frames.

Changed

• TourConfig default mk is no longer related to JUICE CReMA 5.0 and set to None:

TourConfig(kernels=['kernel_1.lts', 'kernel_2.ck', ...])  # `mk=None` is no longer required

• Kernel(s) in TourConfig can be appended with python set() or generator:

TourConfig(kernels={'kernel_1.lts', 'kernel_2.ck', ...})  # Warning: python set don't keep ordering.

TourConfig(kernels=DATA.glob('*.ck'))

• Replace docs/ and examples/ JUICE events resources from FTP to HTTPS.

Fixed

• Downloads in metakernel with duplicated kernels is now fixed. Close #51 (closed).