imp: adds formula primitive type

[lizgehret]

Add primitive type in q2-stats and the framework for the following (currently in q2-composition under ANCOM-BC method):

def _parse_terms(formula):
    parse = formulaic.parser.parser.DefaultFormulaParser(include_intercept=False)
    terms = parse.get_ast(formula=formula).flatten()
    formula_terms = _leaf_collector(terms)
    return formula_terms

def _leaf_collector(term):
    if isinstance(term, formulaic.parser.types.Token):
        return [str(term)]

    if type(term) is not list:
        return []

    return _leaf_collector(term[1]) + _leaf_collector(term[2])