code review changes for miniapp

- improve robustness of time step and time integration
  interval selection
- changes of the type `if (!id)` to `if (id==0)`.
- fix bug selecting source cell gid when generating
  network connections
- make all-to-all network a user-specified option
    - updated README
- fix bug where an initial spike was not generated for
  the first cell on a communicator when the cell gid
  was a multiple of 20