Reduce even more GPU allocations

[thorstenhater]

Bubbling up ion access information from modcc allows us to skip allocation of Xi and Xo iff
no mechanism reads those values.

A minor problem: Sampling may try to touch data that doesn't exist; however who'd ask for Xi
if they never use it?

Stacking this PR on top of #2393 the memory consumption drops by a further 5% down to 77%
if the original value.

Todo

• Guard against sampling non-existing Xi/Xo

[jlubo]

I tested this analogously to PR 2393 and also here, all my tests pass. For the specific case, however, the further reduction in memory consumption is quite small (from 111.5 MB to 109.8 MB).

[thorstenhater]

Well, that's expected as this reduction is conditional on certain commonly used things not being used.

[jlubo]

With many neuronal compartments, I just encountered very high main memory usage, but I guess that is a separate issue.

Regarding this PR, I don't have any further complaints.

[thorstenhater]

The high memory use is a concern, yes, let's discuss that elsewhere, though.