Replace "-" by "_" in directory names

This is a proposal to replace dashes (`-`) by underscores (`_`) in directory names (e.g. `nidm-results` => `nidm_results`).

This will facilitate the re-use of python code included in sub-directories. 

In particular the python files in `test` under `nidm-results` are needed by the FSL export tool and the following import fails because of the dash:
```
from nidm.nidm-results.test.TestResultDataModel import TestResultDataModel
```