Deprecate state point backup files.

[bdice]

Description

Following conversation on #197, I am creating this PR to deprecate methods for state point backup files.

#197 (comment)

Once this PR is merged into master, we should merge master into next. Then we can tackle changes from this PR (removing read_statepoints, write_statepoints, dump_statepoints and some corresponding changes in behavior) in a new PR to next.

Motivation and Context

Reducing the API to core features for signac 2.0 in alignment with discussion on #197.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

[codecov]

Codecov Report

Merging #579 (f2f2975) into master (378a6be) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #579   +/-   ##
=======================================
  Coverage   78.26%   78.27%           
=======================================
  Files          65       65           
  Lines        7066     7069    +3     
  Branches     1310     1310           
=======================================
+ Hits         5530     5533    +3     
  Misses       1231     1231           
  Partials      305      305           

Impacted Files	Coverage Δ
signac/contrib/project.py	85.29% <100.00%> (+0.05%)	⬆️

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[vyasr]

LGTM