Address unsupported operand type(s): 'decimal.Decimal' and 'float'

[rafaelweingartner]

This patch address a situation around mixed type operations. For instance, the following expression, "(1 + 3 + 2.0 + 6) * 0.00001", when evaluated, causes the following error:

>>> from mathparse import mathparse
>>> mathparse.parse("(1 + 3 + 2.0 + 6) * 0.00001")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
File "/home/rafael/git/mathparse/mathparse/mathparse.py", line 792, in
parse
    tokens = tokenize(string, language)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/rafael/git/mathparse/mathparse/mathparse.py", line 570, in
evaluate_postfix
    total = to_number(a) + to_number(b)
            ~~~~~~~~~~~~~^~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for *: 'decimal.Decimal' and
'float'

This patch normalizes the "to_number" operation, to always return Decimal objects. Therefore, the error will not occur, by mixing Number types.