[sombrero] Simplify classes used in benchmarks

[giordano]

Address #237 (comment). @mirenradia Can you please check this solves the issues you reported in the first post?

[mirenradia]

This resolves all of the issues I mentioned in #237 except for the num_tasks problem and I can now run the ITT-sn benchmark (after manually setting num_tasks appropriately). However, I do get the following warning:

WARNING: skipping test 'SombreroBenchmarkScaling': test has one or more undefined parameters

[giordano]

I can reproduce that, as far as I understand it comes from the fact case_filter.generate(scaling_config) in

excalibur-tests/benchmarks/apps/sombrero/sombrero.py

Line 103 in 40b4f4a

params = parameter(case_filter.generate(scaling_config))

is an empty list [], and so the parameter params is undefined. I'm not entirely sure how that benchmark is supposed to work, it was contributed by @mmesiti, maybe he can shed some light on this?

In any case I'm going ahead and merge this PR, as it addresses the previously reported issues.