Fixes rpointer archiving

[jasonb5]

Description

When using the --copy-only or --last_date option, the latest rpointer files would be moved from the run directory breaking the ability to restart a run. The first archived rpointer file would be incorrect as CIME would move the current rpointer into the archive directory and then create all subsequent rpointer files (these would be correct).

• Fixes latest rpointer files being moved from run directory
• Fixes first archived rpointer file contents
• Refactors `_archive_rpointer_files
• Adds unittests

Checklist

• My code follows the style guidlines of this proejct (black formatting)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that excerise my feature/fix and existing tests continue to pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding additions and changes to the documentation

[jedwards4b]

I suggest removing the three unused routines:
get_entries
get_entry_info
get_rpointer_contents.

[jedwards4b]

I could not find any usage of any of get_entries, get_entry_info or get_rpointer_contents in CESM.

[jasonb5]

I could not find any usage either, I'll go ahead and remove them.

[jedwards4b]

Regarding the test failures I found that a check of the object 'archive' at line 200 of case_st_archive.py
reveals it to be of type: <CIME.XML.archive.Archive object at 0x7f40e60e9f90>
not of type env_archive as expected.

[jedwards4b]

Running case.st_archive --test-all reveals a problem with your new code. The
archive variable is of type Archive and not env_archive as you expect.

[jasonb5]

@jedwards4b Thanks, I see the issue. I'll adjust the code and have you review again.