LudvigOlsen/xpectr: xpectr 0.2.0

Adds initializeGXSFunctionAddin() addin for initializing a gxs_function() call for a selected function.

Adds initializeTestthatAddin() addin for inserting testthat::test_that() chunk.

gxs_function() gets argument extra_combinations for manually adding extra combinations of argument values. In some simple cases, this can help us avoid multiple calls to gxs_function() with different baseline values.

The Changed from baseline: comment adds the changed value when only one argument was changed. This makes it faster to see what is tested.

Tests are now properly ordered as the specified args_values.