Intelligently load spice kernels

[ortk95]

Currently we load the same set of spice kernels every single time a Body is initialised - this has a number of issues:

1. Loading kernels isn't instantaneous (~0.3s)
2. Creating a large number of bodies in the same session causes a SpiceNOMOREROOM error
3. Re-loading the same kernels is generally inefficient and messy

Possible fixes:

• Dynamically check if desired kernels are already loaded
• Add some flag somewhere to indicate kernels are already loaded
  • This could potentially cause issues if the user unloads kernels, so would need to document this somewhere
• Add separate function to load the kernels somewhere

[ortk95]

Added flag in ac09b2b - this is probably an improvement, but may cause issues if user manually loads/unloads kernels