ensure_spice functionality for pointing kernel generation

[subagonsouth]

Algorithm Description:

Furnishing of kernels for the pointing (cough, despun) kernel generation code requires unique handling. This ticket is to implement that logic or make sure existing logic correctly handles this case.

Requirements:

• I think that we only want to furnish the single CK that the pointing kernel is being generated for.
• We should identify the minimum set of other kernels that need to be furnished. For example, do we even need sclk or leapsecond kernels?

Ideas for implementation:

• One possibility is to add another keyword argument to the ensure_spice decorator to handle the pointing kernel case.
• Another possibility is to just make sure that the needed kernels are furnished before calling the function that makes the kernel.