Fix/make project config immutable

[vyasr]

Description

Makes the project config immutable. Note that I had to make the _mutable attribute set after construction because the constructor of ConfigObj calls __setitem__ internally, so constructing the ProjectConfig object would always throw warnings (or error in the future). I'm open to alternative suggestions on how to resolve that.

Motivation and Context

Resolves #81, supersedes #244

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.

[codecov]

Codecov Report

Merging #246 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
+ Coverage   64.89%   64.96%   +0.06%     
==========================================
  Files          39       39              
  Lines        5569     5580      +11     
==========================================
+ Hits         3614     3625      +11     
  Misses       1955     1955

Impacted Files	Coverage Δ
signac/contrib/project.py	90.96% <100%> (+0.14%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c21905d...a04fa1e. Read the comment docs.

[bdice]

LGTM.

[csadorf]

I've added a few straight-forward suggestions, but otherwise it looks good to me! Thank's a lot!