Add API to get fluid properties

[aprilnovak]

Reason

There are a number of different base classes in the fluid properties module that the fluid property user objects derive from - such as SinglePhaseFluidProperties and TwoPhaseFluidProperties. Various MOOSE apps may place specific requirements on which of these fluid property options are valid - for instance, Pronghorn restricts fluid properties to those deriving from SinglePhaseFluidProperties. However, to fully check that a user does not try to use a different fluid property base in Pronghorn, we need to downcast a FluidProperties * userobject in each and every Pronghorn class that uses fluid properties to make sure that the correct type was provided:

const FluidProperties * what_user_provided = getUserObject<GeneralUserObject>("fluid");
const SinglePhaseFluidProperties fluid = dynamic_cast<const SinglePhaseFluidProperties *>(what_user_provided);
if (!fluid)
  mooseError("Pronghorn needs SinglePhaseFluidProperties");

This is tedious and open to errors if you forget to perform this check in new classes. Further, the downcasting makes us use pointers, whereas MOOSE would like all MOOSE-based apps to use references whenever possible.

Design

Add a free function like getFluidProperties<T>("name") that automatically checks whether the correct fluid property type has been provided by the user, and error if not.

Impact

Better error checking and protecting Pronghorn users from accidentally using fluid properties that are incompatible with Pronghorn. Benefits to all other applications using fluid properties as well.

[andrsd]

Let's go with the other option, i.e. creating a class for this, so the design would be:

class FluidPropertyInterface {
public:
  FluidPropertyInterface(const MooseObject * moose_object);
  const T & getFluidProperties(const std::string & param_name);
};

The c-tor will save the reference to the MOOSE object, which we will then use to get the object name when reporting errors.
getFluidProperties will check the result of the downcast and report an error if the passed object in not of the requested type.

The impact on code interested in using this will be:

1. add the interface class,
2. change the getUserObject() call to getFluidProperties class.
3. add the call to the FluidPropertyInterface::FluidPropertyInterface