Autoregister modules

Right now we have the load_mm() functions which look like:

mm.add_module(...);
mm.add_module(...);

This article gave me an idea for how to autoregister modules. Basically in SDE we declare some global map std::map<T, ModuleBase*()> factory (T likely typeid(DerivedClass), but maybe have the user set a string). We then define a macro:

#define REGISTER_MODULE(TypeOfModule, key)\
   struct Register ## TypeOfModule {\
       Register ## TypeOfModule(){ factory[key] = [](){ return new TypeOfModule();};}\
   };\
  Register ## TypeOfModule p

which a user puts in their source file like REGISTER_MODULE(MyModule, "DefaultKey");.

Start by reading the existing load_mm() functions and their explicit mm.add_module(...) calls, then compare the proposed factory-map and REGISTER_MODULE approach with the current module-loading design. Done means modules can be declared for autoregistration so load_mm() no longer needs each module added explicitly; the key and factory mechanism still need to be settled.