Allow _chunk_number to be list or tuple

[dachengx]

What is the problem / what does the code in this PR do

This is helpful when you just want to process several chunks for testing.

_chunk_number will be propagated from get_iter:

strax/strax/context.py

Line 1481 in bb23240

_chunk_number=None,

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

coverage: 90.429% (-0.08%) from 90.507%
when pulling d264b25 on chunk_number_list
into bb23240 on master.

[yuema137]

It's straightforward and looks good. Thanks.