Changes

[T-Nicholls]

• Fixed the dependency issues caused by the latest version of pytest-cov.
• Updated load_csv.load() to throw a LatticeException, instead of an ImportError and print statement, when importing cothread fails.
• Added tests for load_csv.load() when control_system=None, to test the default control system (cothread) loads correctly and test that a LatticeException is raised if cothread fails to import; this boosted the test coverage of load_csv to 100%.

[T-Nicholls]

Ready to merge.