archivist no longer loads whole files into memory.

They are now extracted directly to the target location.
@review_id:41696
Added a method to tag a cache as complete.

This is done manually, or automatically when uploading the cache to a
remote directory.

If this file is not present, a cache is considered incomplete (i.e.,
CacheExists will return False)

EDEN-637
Fixed typo
Merge pull request #13 in ESSS/ben10 from EDEN-637 to master

* commit '21fce2e9f03c7e2fc73b41c6002dcc4cb30dde02':
  Fixed typo
  Added a method to tag a cache as complete.
Reverted some bad changes from ETK
Updating with some changes made in coilib50
Removed SkipIfImportError fixture in favor of pytest.importorskip
Adding filename to error description when file is too big
Merge pull request #14 in ESSS/ben10 from fb-improve-terra-former-message to master

* commit '45f1be130c061da236c5cc4f04c09d76517bda1a':
  Adding filename to error description when file is too big
Dircache now allows empty caches

EDEN-674
Ignoring istead of raising an error when uploading a remote cache that already exists
Changed git.Clone 'update if already exists' behavior to always use branch
'master'

EDEN-684
Merge pull request #16 in ESSS/ben10 from EDEN-684-dist-download-not-in-branch to master

* commit '95c59eeabe634da72d86045b47566824ecd7c70e':
  Changed git.Clone 'update if already exists' behavior to always use branch 'master'
Code review: docs and pytest standards. @review_id: 41616.
Code review: pytest standards, docs indentations and empty lines. @review_id: 41617.
Code review: docs. @review_id: 41615.
Code review: Empty lines.
BEN-30: New test for singleton: threads.
BEN-30: Adding ben10.property_.