Setting an offset of an sampled dimesions

So I run into this problem if I want to convert a NIX file to a neo object that the Neo implementation expects an offset in the sampled_dimension for the package quantity to function properly.

This breaks even if you set the offset = 0 in the DataArray.append_sampled_dimension method. It does not add an offset because of this check

~~~python
if offset:
smpldim.offset = offset
~~~

my solution for this problem is simply set the default offset value from None to 0 and remove the check.

Start at DataArray.append_sampled_dimension and inspect how its offset argument is handled when an explicit zero is passed. Verify the behavior around the default value and the conditional assignment, then confirm that a zero offset is preserved and the resulting NIX-to-Neo conversion supports the package quantity as expected.

Written by the indexing model from the issue text.