Polished plotting tools a bit

- Merged them into one file (tools/plots.py);
- Added option to dump plotting data to a text file suitable for using in
  third-party utilities;
- get_e0() now gives tuple (t, E_0(t));
- get_field_distr no longer accepts functions to apply along axis. It should be
  done using matplotlib or manually;
- Some changes to plot captions.