Domain coordinates should be real (#156)

At the moment the domain coordinates are Sympy symbols but we don't
specify them to be real.

Consequently when we take the real part of a complex expression of the
coordinates, the resulting symbolic expression includes the imaginary
part of the coordinate and eventually Pyccel throws an error.

Besides, setting the symbols to real simplifies considerably the
expressions.

---------

Co-authored-by: Martin Campos Pinto <[email protected]>

sympde/topology/domain.py

@@ -750,7 +750,7 @@ def __new__(cls, name, dim, min_coords, max_coords):
             raise ValueError("Min coordinates must be smaller than max")
 
         coord_names = 'x1:{}'.format(dim + 1)
-        coordinates = symbols(coord_names)
+        coordinates = symbols(coord_names, real=True)
 
         # Choose which type to use:
         #   a) if dim <= 3, use Line, Square or Cube;

sympde/topology/tests/test_gallery.py

@@ -4,7 +4,7 @@
 from sympy import Tuple
 from sympy import symbols
 
-x1, x2, x3 = symbols('x1, x2, x3')
+x1, x2, x3 = symbols('x1, x2, x3', real=True)
 
 from sympde.topology import Interval, ProductDomain, InteriorDomain, Domain
 from sympde.topology import Line, Square, Cube, NCubeInterior