Fixing relative path loads in `io`

[sneakers-the-rat]

While i was seeing if the 'data' folder was being used in the code, i found that there was a place in io where a relative path was used that would fail if the package wasn't being run from the repository root (ie. in cases where one installs from PyPI or etc.). The real way to do this would be to have a CALIBRATION_DIR in prefs that allows the user to set where their calibration files are, but this PR does some basic path safety stuff:

• if given an absolute path, use it
• if given a relative path that can be resolved to being from the root of the repository/directory containing 'pvp', do that (currently paths are given as pvp/io/... so not quite a relative path from the root of the package).
• if neither of those are true, try to load the relative path
• if that fails, raise an exception with informative error message

before i merge i wanted to make sure with y'all that this doesn't break the actual system

[mschottdorf]

makes sense & looks good to me

[mschottdorf]

looks good to me; ready to merge

[sneakers-the-rat]

ok pulled changed from master to this branch and will merge if tests pass