add unit tests for to_respilens()

[emprzy]

3 unit tests that check:

1. that to_respilens() will fail if the parameter is not an acciddacast type
2. that to_respilens() will fail if there is >1 location in either of the acciddacast objects
3. that the output is the correct hierarchical structure (nesting check)

Also I fixed the misspelling of "accida" issue in this PR (issue #53)

@CyGei let me know if you want me to add more tests or take some away.