Make code generation robust by using "safe" variable names

[yguclu]

When we generate the AST for assembly of scalars (norms), vectors (linear forms), and matrices (bilinear forms), we always create some new symbolic variables, which are usually elements of a given function space. In order to avoid unexpected behavior, we must choose names for these variables that are not already used in the user-defined expressions.

Unfortunately, so far we have used the name Nj to identify basis functions, and w for NURBS weights, without performing any check. This will raise issues if the same names Nj and w are already used in the expressions to be evaluated.

See https://github.com/pyccel/psydac/pull/51#discussion_r340212209.