feat[cartesian]: Specify compiler optimization level and flags via env variables

[stubbiali]

Description

This PR allows downstream applications to specify compiler optimization level and flags on a stencil basis.

Requirements

• All fixes and/or new features come with corresponding tests.
• Important design decisions have been documented in the approriate ADR inside the docs/development/ADRs/ folder.

If this PR contains code authored by new contributors please make sure:

• All the authors are covered by a valid contributor assignment agreement provided to ETH Zurich and signed by the employer if needed.
• The PR contains an updated version of the AUTHORS.md file adding the names of all the new contributors.

[havogt]

Please add a test that illustrates using the feature. Not sure if removing debug_mode is a good idea.

[havogt]

Adding @egparedes and @FlorianDeconinck for more opinions.

[FlorianDeconinck]

LGTM

[stubbiali]

@havogt On (my) mac, the flag -DNDEBUG is required to make compilation works with any optimization level, so I hardcoded it into mode_flags for the sake of convenience. If you prefer, we could be more flexible:

Suggested change

	mode_flags = [f"-O{opt_level}", "-DNDEBUG", *extra_opt_flags.split()]
	mode_flags = [f"-O{opt_level}"]
	if extra_opt_flags != "":
	mode_flags += [*extra_opt_flags.split()]
	elif opt_level == "0":
	mode_flags.append("-ggdb")
	else:
	mode_flags.append("-DNDEBUG")

[havogt]

cscs-ci run