Host timing

[neworderofjamie]

When using custom connectivity updates with host code strings, there was previously no way of measuring their performance. This PR moves the std::chrono based Timer helper class from the single-threaded CPU backend into codeGenUtils for general use and uses this to implement a new per-custom update timer for this code. This can be accessed from C++ via Runtime::getCustomUpdateHostTime and from Python via GeNNModel.get_custom_update_host_time

Also, based on user feedback, tweaked the documentation to clarify that the 'name' passed to the GeNNModel.get_custom_update_XXX_time methods is the name of custom update group not an individual custom update.

[codecov]

Codecov Report

❌ Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.39%. Comparing base (80dc278) to head (016c2e4).
⚠️ Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
src/genn/genn/code_generator/codeGenUtils.cc	77.77%	2 Missing ⚠️
pygenn/genn_model.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #701      +/-   ##
==========================================
- Coverage   88.75%   88.39%   -0.37%     
==========================================
  Files         106      109       +3     
  Lines       14180    14448     +268     
==========================================
+ Hits        12586    12771     +185     
- Misses       1594     1677      +83     

☔ 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.

[tnowotny]

It's a bit deep in, but I think I get it, and it is pretty straightforward refactoring.