Useless/misleading stage instance debug_mode attribute?

[thehrh]

This attribute claims to disable caching at the Stage base class level, but it doesn't actually seem to have any functionality. No service throughout the code base implements any custom behaviour.
debug_mode was introduced in 4cb9172 and used to have various effects as one can tell.

What to do with it? Make it, e.g., force compute always to call compute_function at least (no caching on identical parameter value hashes)?

[thehrh]

I suggest keeping this attribute while removing the claim that it has any generic effect on all stages for now (see #864). Such generic behaviour could still be added on top later if need be.