Improve iterators and restore outputlevel

[emstoudenmire]

This PR introduces a handful of improvements to the updated solvers codes:

• Add unit tests for SweepIterator
• Fix issues with SweepIterator, including the case of 1 sweep (see Fix bug with length 1 AbstractNetworkIterator types not executing their only compute! call.  #262) and fixing length for SweepIterator
• Support for outputlevel added back to eigsolve and applyexp
• The nsites keyword is no longer required to construct a SweepIterator (was accidentally required by not providing a default in region_plans/euler_plans.jl.

This PR supersedes #262 by including all changes from it.

[codecov]

Codecov Report

❌ Patch coverage is 68.42105% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.54%. Comparing base (beed105) to head (9e64295).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/solvers/eigsolve.jl	33.33%	4 Missing ⚠️
src/solvers/iterators.jl	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #267      +/-   ##
==========================================
+ Coverage   68.53%   68.54%   +0.01%     
==========================================
  Files          81       81              
  Lines        4023     4031       +8     
==========================================
+ Hits         2757     2763       +6     
- Misses       1266     1268       +2     

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

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.

[mtfishman]

I'll close this since the most pressing issue (the outputlevel issue) was addressed in #268, and we can discuss and work on the other parts separately.