#49 Clean up and unify the propagators

Three tasks actually:

• clean up the OperatorMatrix. Basically, update it, add required functions etc., so that it feels easy and natural to use in practice.
• join Propagators and MultiPropagators. Basically, everything should be a MultiPropagator. However, the setup should be reasonably simple, if necessary also optimized, for problems where you would normally use a Propagator.
• Update the PropagationManager/Observer(s) to work with MultiPropagators.

[Anonymous]

Chat summary:

• OperatorMatrix gets a new constructor that takes a single operator
  and sets up a 1x1 matrix with this operator (to handle the trivial
  case)
  • invalid_argument exception if the operator is null
• OperatorMatrix gets a new constructor that takes a vector of vectors
  of operators (rows in the matrix), and sets up the operator
  • invalid_argument exception if all operators null
  • invalid_argument exception if matrix not quadratic
  • invalid_argument exception if wrong number of operators in a row
• add a new static variable that represents a null operator (null
  pointer); supplying this operator in the second constructor signals
  that this cell of the matrix is the zero operator.
• add a function getSize() to return the size of the OperatorMatrix
• add a function getOperator(uint row, uint column) that returns the
  appropriate operator in this cell of the matrix

[Anonymous]

Chat summary:

• Add time test in OperatorMatrixTest
  • set up a simple functor (t -> t) and use TimeFunction as a test operator
  • set up a testing matrix and check that the operator is applied correctly
• MultiPropagator gets a convenience function that passes a single state
  • throws exception if the OperatorMatrix is not a 1x1 matrix
  • otherwise, processes this as a size-1 vector
• PropagationManager takes a MultiPropagator instead of a Propagator
  • retain convenience function that takes a single state
  • add function that takes a vector of states
• PropagationObserver takes vector of states
  • update the LoggingOutput to do something sane; whatever that is
• update demos
• replace Propagator everywhere by MultiPropagator
• remove Propagators, rename MultiPropagators to Propagators