Add variable_mapping table to NEON mapping functions

[clnsmth]

The variable_mapping table holds relations between variable names in the L1 tables and measurement definitions in external vocabularies and ontologies. Adding this table to L1 datasets will facilitate improved data search and integration.

Currently, NEON data products converted to an L1 ecocomDP dataset do not include this optional table.

Within the NEON system, many measurement variables are already mapped to external vocabularies and ontologies. Additionally, this information is available via the NEON API. Therefore, we should be able to add the variable_mapping table as an extra step in the NEON mapping functions.

Thoughts @sokole?