Get Kernels from MKs (with path correction to SPICEROOT)

[jrcain-usgs]

When Metakernels were auto-loaded from SPICEROOT, they would often still have their default directory, /usgs/cpkgs/isis3/data, which does not exist on most user's systems, and must be manually changed by the user.

This PR checks auto-loaded metakernels for correct kernel paths (in both the mk's default paths, and SPICEROOT), and returns a list of kernels paths that exist, or fails with an error explaining which kernel is missing from where.

Fixes #529

Licensing

This project is mostly composed of free and unencumbered software released into the public domain, and we are unlikely to accept contributions that are not also released into the public domain. Somewhere near the top of each file should have these words:

This work is free and unencumbered software released into the public domain. In jurisdictions that recognize copyright laws, the author or authors of this software dedicate any and all copyright interest in the software to the public domain.

• I dedicate any and all copyright interest in this software to the public domain. I make this dedication for the benefit of the public at large and to the detriment of my heirs and successors. I intend this dedication to be an overt act of relinquishment in perpetuity of all present and future rights to this software under copyright law.

[Kelvinrr]

Needs changelog and tests for new function.

[oleg-alexandrov]

I added the semi-related #705. I had some fix for this particular issue as well but then took it out not to have a conflict. That one addresses something else.

All these show up with Chandrayaan-2. This particular thing is very long-standing.

[acpaquette]

Initial review

[Kelvinrr]

We should figure out how to integrate this into the MK ale.loads paradigm. So users dont have to use python to make it work.

[oleg-alexandrov]

@Kelvinrr Thanks a lot for the review and @jrcain-usgs for the work. I validated that this works. As to integrating this into the MK ale.loads paradigm, that would likely be nice, but the fix does what it intends. When going through the NAIF path the metakernel gets loaded properly. The other path is with -k when an ISIS cub is loaded and meta kernels are not needed. I can't think of a use case where the bug would still be triggered (so unless the user does something custom).