Chunk interval index

[jmosbacher]

Implementation of #433, lazily constructs an IntervalIndex for chunks. The index can be used for fast overlap tests
also added a overlaps method to strax.Chunk which returns all data that overlaps with a given interval.
Can you give a minimal working example (or illustrate with a figure)?

import strax
import straxen

st = straxen.contexts.demo()
c = next(st.get_iter( '180423_1021','raw_records',))
idx = np.random.randint(len(c.data))
row = c.data[idx]
overlapping_data = c.overlaps(row['time'] , row['time'] + row['dt'])

[coveralls]

Coverage decreased (-0.2%) to 85.736% when pulling 210b923 on jmosbacher:chunk_interval_index into d3608ef on AxFoundation:master.