FIX: preserve non-sympy arguments in func

[redeboer]

Closes #524

🐛 Bug fixes

• Classes decorated with @unevaluated now override func whenever they have fields declared with argument(sympify=False). Before, SymPy operations that rebuild an expression through expr.func(*expr.args) (replace, rewrite, doit, cse) silently reset these arguments to their defaults. For EnergyDependentWidth with phsp_factor=PhaseSpaceFactorSWave, for example, expr.replace(R, R2) evaluated to a different value than expr.
  • If all non-sympy arguments have their default values, func returns the class itself.
  • Otherwise func returns a subclass that fills in the instance's non-sympy values and constructs an instance of the original class. It compares equal to and hashes like the original class, so checks such as expr.func == BreitWigner, expr.has(expr.func) and expr.find(expr.func) keep working. Rebuilt instances have the original type, so pickling and cache hashes do not change.
• xreplace now replaces a non-sympy attribute only if the rule has a key of the same type. Previously False matched a key 0 or sympy.Integer(0), and mappings like ParameterValues that accept integer positions could replace such attributes by accident.

❗ Behavioral changes

• The default str and srepr printers now render non-sympy arguments that differ from their defaults as keyword arguments, for instance MyExpr(0, 3.14, functor=Transformation). This lets parse_expr reconstruct the original expression, given a namespace with the expression class and any classes used as attribute values. Custom printer methods and LaTeX output are unaffected.

Squash commit messages

* BEHAVIOR: print non-default non-sympy arguments in str and srepr
* FIX: match xreplace rules on non-sympy attributes by type