Add a get_files method

get_files() method returns the full path of the files, matched either by
obsid or time range.  Rename get_interval_files to _get_file_records, and
add an obsid_times method to get start and stop times from archived obspars.
Add a mechanism to track "default" files

For asp_l1 and obspar archives, update the archfiles table to include
an isdefault column and then set that value when the default
version/link is updated for an obsid.
Add a get_files method to obsid_archive

This will be for use in the per-obsid tasks, asp_l1 and obspar.
Add get_files method in asp_l1 and obspar
Add examples to aca_l0 docstrings
Add examples to docs in asp_l1

Add docstring for get_files()
Add docstrings and examples to obspar
Add docstrings to aca_hdr3
Add MSID description text to hdr3 objects
Improve MSID attr passing in aca_hdr3
Fix bug in hdr3 data masking
Use object config for data_root in get_files()

Instead of passing it as an option.
Add logic to get_files by obsid or time range
Sort get_files output by tstart
Add aca_l0 and asp_l1 get_files to docs
Combine module docs in one api page
Remove individual module docs
Copy module examples to top level docs
Put some utility scripts in 'utilities'
Fix docstring typo