Added a light-weight Airviro result reader and writer.

- The pyAirviro.disp.res.ResultReader class is used to read exported
  Airviro results, e.g. the output from the `exportres' command.
- The pyAirviro.disp.res.ResultWriter class is used to write new
  Airviro results that could be read by e.g. the `importres' command.
- Unit tests are included in the pyAirviro.tests.disp.test_res module.
- A little time was spent optimizing the IO. The bottleneck is mainly
  reading the text input into numpy arrays.
- This commit was made in connection to adding the subgrid command to
  the Airviro tools.

I do these pull request mostly to spread the information, but code reviews are of course very welcome if you don't have enough to do anyway ;) I will do the merge in a day or two, unless you stop me.