Static object which registers Plumed.
This is a static object which, during its construction at startup, registers the pointers to plumed_plumedmain_create, plumed_plumedmain_cmd and plumed_plumedmain_finalize to the plumed_kernel_register function. Registration is only required with plumed loader <=2.4, but we do it anyway in order to maintain backward compatibility. Notice that as of plumed 2.5 the plumed_kernel_register is found using dlsym, in order to allow the libplumedKernel library to be loadable also when the plumed_kernel_register symbol is not available.