Draft: New Sampling tool interface, sampling generator components, and PileupTool for rare processes

• Adding a new ISamplingTool interface
• Adding Special and MinimumBias sampling generator classes in Gaussino.
• Adding a new FixedLuminosityForRareProcessWithSvc Pileup tool for rare processes to be used with Madgraph.
• A new adapted GsinoExternalGenerator is also added.

All these changes are up to date with the new generator interfaces as per !151 .

A more detailed explanation with diagrams, and on how to test it can be found here https://codimd.web.cern.ch/JtD7xAGqS6S9eSIyfmKqRA#.

Relates to #39