Improve subgraph from QuotientVertex and other small improvements

[mtfishman]

With this PR, subgraph(g, QuotientVertex(v)) now outputs an unpartitioned graph, while before for PartitionedGraph it output a PartitionedGraph with one partition.

Additionally, this makes a small fix to has_edge when the edge involves vertices that aren't in the vertex list of the graph, and also some small improvements to minimize ambiguities when overloading the interface.

This all helps with getting ITensor/ITensorNetworks.jl#264 merged.

[codecov]

Codecov Report

❌ Patch coverage is 85.29412% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.53%. Comparing base (d8ba480) to head (51b6f96).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lib/PartitionedGraphs/src/quotientvertex.jl	70.00%	3 Missing ⚠️
.../PartitionedGraphs/src/abstractpartitionedgraph.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
- Coverage   85.66%   85.53%   -0.13%     
==========================================
  Files          54       54              
  Lines        1765     1777      +12     
==========================================
+ Hits         1512     1520       +8     
- Misses        253      257       +4     

Flag	Coverage Δ
docs	0.00% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.