Good bye sympy dummy attribute

[mmatera]

This continues #1533. Now, let's remove the sympy_dummy attribute for symbols. When it is necessary (or useful) like in Sum or Product expressions, we can tell to to_sympy which symbols should be handled as sympy dummy symbols.

[mmatera]

@rocky, you have revised #1536 and #1534. This is just remove the sympy_dummy attribute from the Symbol class. Are you OK with this, too?

[rocky]

@rocky, you have revised #1536 and #1534. This is just remove the sympy_dummy attribute from the Symbol class. Are you OK with this, too?

Yes, thanks!