[v3] Config check is incomplete and broken

[denisri]

Config modules dependencies are not checked properly for execution: a config module can declare it depends on another, for instance spm depends on matlab, but the validity of this dependency is not checked and cannot be checked at execution time, and dependent modules are dropped from the execution context config.

• In our example, a process may depend on spm, but will not receive a matlab config if it doesn't declare it explicitly, which should not be needed.
• the validity of each module config is checked independently and without access to other (dependent) modules config: here, the spm module is_valid_config() method only receives the spm module config, and cannot check if the matlab module is configured appropriately (for standalone or matlab execution, typically).
• As a consequence, the SPM module, configured for instance with standalone=False option, cannot require matlab with a valid directory, or SPM with standalone=False, cannot require matlab with a valid mcr_directory.

The current API does not allow to configure things correctly, so it must be reworked.

[denisri]

ModuleConfiguration.is_valid_config() could return additional requirements instead of just True/False For instance SPMConfiguration.is_valid_config() could return {'matlab': {'mcr': True}} or {'matlab': {'mcr': False}} depending on the SPM standalone state, which would add both dependency and constraints on the matlab modules (in the same way/syntax as Process.requirements). Those added requirements would then be checked (again?) to see if they match.