Refactor preamble code.

[1uc]

The functions that serve as "entry points" into the MOD file, need to set up some variables that we then use, e.g.

auto _lmr = ...;
auto inst = ...;
auto node_data = ...;

This code is repetitive, but seems to be done adhoc in the NEURON code printer. It would be nice to make it systematic. We need to distinguish at least the following:

• access to the memb_list vs. only a _prop.
• access to a node (i.e. everything that's not an ARTIFICIAL_CELL).
• access to thread data in two favours: a) any access at all i.e. VECTORIZED==1, b) access to the NRN thread specific copy of _thread.