Refactor CLI Kernel Arguments

The kernel arguments are currently passed as unnamed parameters following all other CLI arguments to star. This is a bit unintuitive, and could be clarified significantly with a few design changes.

Proposal:

• Make kernel_args a named argument in the CLI
• kernel_args should be a list of floats or a dictionary that maps argument names to floats
• star should throw more explicit errors when mandatory kernel arguments are not correctly provided.