extractSMR: add support for soil moisture subclasses

[brownag]

Result when specifying an additional argument (i.e. subclass=TRUE) would be a data.frame with soil moisture regime and soil moisture subclass columns.

Originally posted by @brownag in #48 (comment)

For the future: dealing with parsing soil moisture subclasses via an additional option would be a great new feature, but they have more complex rules depending on the order/suborder they occur in. e.g. Aridic Mollisols have the subclass in the suborder. Currently extractSMR() is not aware of these types of relationships within taxon names and would need a more detailed set of rules or lookup table to handle these cases.

To implement need to recognize which level each formative element is occurring in input taxon name with proper handling for formative elements that can occur at multiple levels.

• Likely would benefit from refactoring approach in extractSMR().
• Also would likely benefit from caching SMR and SCL information in ST_formative_elements as new or existing columns RE: design formative element dictionary content, goals, editing suggestions #17