Replace _is_like_identity_operator and _is_like_zero_operator

[sdrave]

After merging #464 we should either get rid of these helper methods or replace them by something better. These are used to check E for identity and D for zero-ness in LTISystems. Possible options that come to mind:

• Ensure that these operators are always exactly ZeroOperator or IdentityOperator by modifying LTISystem.__add__ and SecondOrderSystem.to_lti.
• Move the methods to pymor.algorithms and base them on RuleTables.
• Add pymor.algorithms.simplify which replaces such operators by IdentityOperator or ZeroOperator.
• Add similar functionality to pymor.algorithms.preassemble.

[pmli]

Another possible option is

• Using None instead of IdentityOperator and ZeroOperator for E and D.

One issue with forcing IdentityOperators is that there should be a way to assemble IdentityOperator + BlockOperator, e.g., as with E and A in LTISystem.

[sdrave]

After fixing #485, special handling of D should no longer be necessary.