added about_equal function for sqlalchemy

[dmtvanzanten]

I created this PR mostly to share my code and not so much as to have it actually implemented into the master. I the knowledge to write the same implementation for the SQL and Django backends and I realize that that is a requirement for this functionality to be considered. I hope someone will be able to add this functionality for those backends as well.

For now I will just proceed using

from odata_query.grammar import ODataParser, ODataLexer, ODATA_FUNCTIONS
from odata_query.sqlalchemy.common import _CommonVisitors
from odata_query import ast
from sqlalchemy import func
from sqlalchemy.sql import functions
import operator

ODATA_FUNCTIONS['aboutequal'] = 3

def func_aboutequal(self, field: ast._Node, value: ast._Node, tolerance: ast._Node) -> functions.Function:
    ":meta private:"
    field = self.visit(field)
    value = self.visit(value)
    tolerance = self.visit(tolerance)
    return operator.lt(func.abs(operator.sub(field, value)), tolerance)  

_CommonVisitors.func_aboutequal = func_aboutequal