Separate ISimulator interface and data

Separate ISimulator interface and data

In fulfillment of #3566 this separates ISimulator into a pure
virtual interface (still named ISimulator) and a data object
LegacySimulatorData.

The LegacySimulator now inherits from both classes, no other
changes are needed.

ModularSimulator takes the new LegacySimulatorData as an explicit
member object. ModularSimulatorAlgorithmBuilder uses a pointer to
this new member object rather than holding a copy of the
LegacySimulatorData members.

Closes #3566