Improve `ala_counts()` cross tabulation and `select_filters()` parsing

[daxkellie]

Changes to allow multiple arguments and better parsing by select_filters(), select_fields() and ala_counts(group_by = ):

• Improve query building and parsing of arguments (#90, #98)
• Add select_filters() parsing tests (#92)
• Add select_fields() function to parse arguments passed to ala_counts(group_by = ) (#95 )
• Fix select_taxa() bug that builds incorrect queries of searches including parentheses (#96)
• ala_counts(group_by = ) accepts multiple arguments using select_fields() function and cross tabulates results (#99)