Remove references to AoS layout in CoreNEURON.

[1uc]

CoreNEURON used to be able to handle the situation where some MOD files used AoS and others SoA. The functionality was removed here:
BlueBrain/CoreNeuron#675

However, there's still traces left. For example:

nrn/src/coreneuron/coreneuron.hpp

Line 170 in 65767ec

auto& get_mech_data_layout() {

nrn/src/coreneuron/io/mem_layout_util.cpp

Lines 34 to 35 in 65767ec

	case Layout::AoS:
	return icnt * sz + isz;

We can reduce the complexity of the code by removing these. One way might be to remove the method get_mech_data_layout and then let the compiler identify all the locations that need changing.

[mgeplf]

First pass here: #2889
Need to go through more carefully on my end, so no need to look (but feel free).

[1uc]

It's a slightly more involved process, since some code that should be simplified is still used with Layout::AoS.

[1uc]

Second attempt here: #2895.