Topic/plat 6640 part2

This is the bulk of the new caplet stripping code. The idea is to have a system that makes it easy to add new models. Only a few models are implemented here. Note, some tests are marked as TestGroup.UNIT_SLOW, which will reduce the test coverage on systems that do not run these tests.   
It is a bit confusing that `CapletStrippingImp` is named as such - this implies to
me that there is some abstract `CapletStripping` interface, and there will be one
or more implementations of it. However, this is not the case -
`CapletStrippingImp` is a class that contains most of the core functionality
required to do caplet stripping. Perhaps calling it `CapletStrippingCore` 
would make the design clearer?