Generalize the code printing of And, Or, Not.

Move the printing of And, Or, and Not up in the class hierarchy to
the CodePrinter class and make it customizable by overriding a dict.
Add Fortran code printing for And, Or, Not, add tests.
Generalize the code printing of Relational.

Allow relational operators to be overriden using a dict.
Fix the Fortran printing of Ne, add tests.
Implement Fortran printing of Xor, Equivalent, add tests.
Fix the precedence order for Relational.

Relationals are somewhere between And and Add operations
in the precedence tables for python, C, Fortran, etc.
Remove the spaces from the definitions of operators.

Instead, the spacing is done directly by the _print_Op methods.
Merge pull request #2556 from jrioux/printing

Fortran code printing of logical and relational operators.