ASV - Create Basic Benchmark Testing Suite

[trexfeathers]

ASV configuration and a basic benchmark are now part of Iris (#3526). Now need to populate the tests/benchmarking directory with a set of tests that will be able to highlight any critical regressions in Iris' performance as more commits are made.

More tests will no doubt follow in the future, so should focus on the most important areas at first.

• Determine the functions of Iris that are both highly used and highly vulnerable to problematic performance degredation
• Write tests in tests/benchmarking that would highlight regression in the performance of these functions
• Configure the regressions_first_commits and regressions_thresholds sections of asv.conf.json to ensure that changes in performance truly considered regressions are highlighted

[trexfeathers]

This will be a continuous iteration from now on. #4253 at least provides a starting point.