Start to reduce/refactor arithmetic ...

Remove stuff from mathics.builtin.arithmetic that does not belong there:

* _MPMathFunction -> MPMathFunction and move to mathics.buitin.base
* _MPMathMultiFunction -> MPMathFunction and move to mathics.builtin.base
* Abs, Piecewise, RealAbs, RealSign, Sign moved to numeric to follow WMA
  organization better

The corresponding eval routines will be gone over in another PR. This
one is already large.

Url's gone over to make this not exceed standard line limit.
Note that the formatting has been gone over to follow the existing
pattern that we have been using.

cythonization in mathics.builtin class files removed. It is not
clear this has benefit in modern Pythons, especially in this kind of builtin function
and all of this needs to be retested if not rethought.

mathics/builtin/arithfns/basic.py

@@ -2,12 +2,19 @@
 """
 Basic Arithmetic
 
-The functions here are the basic arithmetic operations that you might find on a calculator.
+The functions here are the basic arithmetic operations that you might find \
+on a calculator.
 
 """
 
-from mathics.builtin.arithmetic import _MPMathFunction, create_infix
-from mathics.builtin.base import BinaryOperator, Builtin, PrefixOperator, SympyFunction
+from mathics.builtin.arithmetic import create_infix
+from mathics.builtin.base import (
+    BinaryOperator,
+    Builtin,
+    MPMathFunction,
+    PrefixOperator,
+    SympyFunction,
+)
 from mathics.core.atoms import (
     Complex,
     Integer,
@@ -387,7 +394,7 @@ def eval(self, items, evaluation):
         return eval_Plus(*items_tuple)
 
 
-class Power(BinaryOperator, _MPMathFunction):
+class Power(BinaryOperator, MPMathFunction):
     """
     <url>
     :Exponentiation:
@@ -531,7 +538,7 @@ class Power(BinaryOperator, _MPMathFunction):
     def eval_check(self, x, y, evaluation):
         "Power[x_, y_]"
 
-        # Power uses _MPMathFunction but does some error checking first
+        # Power uses MPMathFunction but does some error checking first
         if isinstance(x, Number) and x.is_zero:
             if isinstance(y, Number):
                 y_err = y
@@ -788,7 +795,6 @@ def inverse(item):
                 and isinstance(item.elements[1], (Integer, Rational, Real))
                 and item.elements[1].to_sympy() < 0
             ):  # nopep8
-
                 negative.append(inverse(item))
             elif isinstance(item, Rational):
                 numerator = item.numerator()