Merge ProblemOperatorBase with ProblemOperator

[EdwardPalmer99]

Brief

• Merge ProblemOperator with ProblemOperatorBase.
• TimeDomainProblemOperator now inherits from ProblemOperator etc.

Diamond Inheritance

• mfem::TimeDependentOperator should inherit from mfem::Operator virtually. Then hephaestus::ProblemOperator would inherit from mfem::Operator virtually and hephaestus::TimeDomainProblemOperator would finally inherit from mfem::TimeDependentOperator and ProblemOperator with no ambiguities (see diamond inheritance on Wikipedia).
• Unfortunately, this cannot happen without modifying mfem. A workaround is for the ProblemOperator to inherit virtually from mfem::Operator as before and for TimeDomainProblemOperator to inherit from ProblemOperator and virtually from mfem::TimeDependentOperator. We then need to add virtual setters for the width/height member variables stored in mfem::Operator.

Other Changes

• Solve, Mult, ImplicitSolve methods now have MFEM_ABORT if they are not overriden preventing user from calling empty methods.
• Solve method inherited from hephaestus::ProblemOperator by hephaestus::TimeDomainProblemOperator has MFEM_ABORT with message explaining that it cannot be used for time-dependent problems.

Design Questions

• Does it make sense for builder classes to implement ConstructOperator method for a problem operator (i.e. ProblemOperator, TimeDomainProblemOperator) class which does not implement Solve/ImplicitSolve methods?