Add new sectors to NAICS crosswalks upon crosswalk load

[bl-young]

Needed for external activitytosectormapping files that may include new NAICS (e.g. 7 or 8 digit NAICS) not currently in the crosswalk.

Crosswalk could be appended locally after loading, perhaps in load_crosswalk()

https://github.com/USEPA/flowsa/blob/aaf54030e701d6c52962eb2439ac3a683eb7c21a/flowsa/common.py#L59-L76

Currently that crosswalk will only include activitytosectormapping files and NAICS within the flowsa repo

[matthewlchambers]

Functionality to add such new "NAICS" codes (or any other industry codes that aren't actual NAICS codes) is in the seea-recursive branch. I needed it to account for emissions from waste decomposition in accordance with SEEA, which says these emissions should be attributed to "accumulations," not to the (current activities of) the waste management industry.

https://github.com/USEPA/flowsa/blob/fc14218d8f9c6445126f39dfdf698174ccd259c4/flowsa/naics.py#L33-L37