local data only arrayset iteration

[rlizzo]

Motivation and Context

Why is this change required? What problem does it solve?:

added ability to iterate over keys, values, items of an arrayset with only locally available data.

Description

Describe your changes in detail:

added optional local argument to Arrayset .keys(), .values(), .items() methods. If set True, only return elements which exist on the local disk (not on a remote server). Default is False, which returns all samples recorded in the commit. This keeps the same behavior as previous uses of these methods.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Documentation update
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Is this PR ready for review, or a work in progress?

• Ready for review
• Work in progress

How Has This Been Tested?

Put an x in the boxes that apply:

• Current tests cover modifications made
• New tests have been added to the test suite
• Modifications were made to existing tests to support these changes
• Tests may be needed, but they are not included when the PR was proposed
• I don't know. Help!

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have signed (or will sign when prompted) the tensorwork CLA.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #131 into master will increase coverage by 0.02%.
The diff coverage is 93.94%.

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
+ Coverage   92.62%   92.65%   +0.02%     
==========================================
  Files          61       61              
  Lines       10520    10608      +88     
  Branches     1025     1041      +16     
==========================================
+ Hits         9744     9828      +84     
- Misses        564      566       +2     
- Partials      212      214       +2

Impacted Files	Coverage Δ
src/hangar/checkout.py	89.63% <ø> (+0.46%)	⬆️
tests/test_arrayset.py	100% <100%> (ø)	⬆️
src/hangar/backends/selection.py	100% <100%> (ø)	⬆️
src/hangar/backends/__init__.py	100% <100%> (ø)	⬆️
src/hangar/arrayset.py	89.05% <79.31%> (-1%)	⬇️

[hhsecond]

Looks good