Add support for multiple network names and channel type

[niyiyu]

This PR contains some small modifications to the PNWDataStore object. After the modification, the PNWDataStore object now supports:

• channel filter for multiple channel types, e.g., loading HH,EH,BH at the same time
• reading data from multiple network name, e.g., UW, UO, CC, at the same time. The later one is achieved straightforward by modifying DATA path: we replace hardcoded network name to "__", which is the wildcard in the sqlite grammar indicating any two chars.

See the updated tutorial (noisepy_pnwstore_tutorial.ipynb) for more information. We are doing more tests at this moment.

[carlosgjs]

Just one suggestion to cleanup the extra cell. Looks good!

[niyiyu]

Thanks for all of your comments. I will leave the switch of the frequency filter in another PR.