nrn_setup: use std::vector for cell_groups (dat_files)

[ferdonline]

Context

Coreneuron nrn_setup uses a plain malloc'd array for holding the input data cell groups (from files.dat). These arrays, besides being plain old and not self managed, could not grow and had a static size of nfiles/nranks + 1.

However, in the view of the upcoming load balanced input data, each rank may hold a different count of cell groups. Those changes will come as a follow-up PR.

This PR

Refactors this part of the code to replace the data structure by an std::vector, which

• grows as required and auto deallocs
• avoids the book-keeping of the size as a separate variable