Define new pull external potential provider RAMD and register it for grompp and mdrun

[BerndDoser]

The PULL code provides an special type which allows to define external potentials. This is used, for example, by the module accelerated weight histogram method (AWH). The external potential type must be defined with ‘pull-coord1-potential-provider’. In the code the new external potential type 'RAMD' must be registered in grompp and mdrun.