Storing only absolute paths for instrument_base_dir

[shervin86]

In calculators there is a member storing the instrument_base_dir.
I think it would be more robust to store it always as an absolute path, even when the user sets it as relative to the current working directory. That would reduce the number of possible issues when paths or the current working directory is changed.
I would suggest to use in the setter

self._instrument_base_dir = os.path.abspath(value)

Then the calculator_base_dir will be relative to the instrument_base_dir