fix indexing of IntervalSet to be able to use -1

[sjvenditto]

addresses issue #407

fixes indexing of IntervalSet such that indexing like ep[-1] or ep[[0,-1]] doesn't throw an error

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
pynapple/core/interval_set.py	94.61% <100.00%> (+0.27%)	⬆️
pynapple/core/metadata_class.py	78.89% <ø> (-3.67%)	⬇️

... and 1 file with indirect coverage changes