Add a MachineManager class.

Add a class for the purposes of managing simulated machines. If
this class is used properly, future commits can help cut down on
the number of checks that occur to each call on ptl_simulate.
Move machine initialization code.

Move machine-specific initialization code out of the ptl_simulate
loop and into a function within the PTLsimMachine class.
Delay the invocation of PTLsimMachine::initialize().
Conditionally build-in test framework stubs.

Prevent "test" code from being compiled and linked into non-debug
builds of MARSS. Rely on the definition (or lack thereof) of the
existing ENABLE_TESTS macro.