Revisit case symlinks

[jasonb5]

This proposal is to collapse all the existing symlinked scripts into a single entrypoint; cime. This will become the only symlink in the case directory. To preserve the current functionality all other symlinks will be replaced with light shell wrappers calling the new entrypoint.

The purpose is to simplify changing which CIME driving the case. Now it's simply updating a single symlink.

All existing scripts will still be available in a case. And existing cases will still work.

Example

The case.setup will simply be a shell script:

#!/usr/bin/env bash
./cime case.setup

[rljacob]

That should work.

[billsacks]

Thank you for this, @jasonb5 ! I like that this will clean things up while still maintaining the same user-facing CLI that users have grow accustomed to.