Reorganize includes

I propose to change the organization of the includes in the source folder and to change the folder statismoConfig.cmake provides as statismo_INCLUDE_DIR. I find it much clearer to specify the library and module name for includes. Also possible confusions of headers with the same name in different modules or libraries are prevented by this change.

The resturctured include directory in the modules sources will reflect the structure after installing. The main change is that the includes get additional subdirectory prefixed:
old: statismo/core/include/Kernels.h
new: statismo/core/include/statismo/core/modules.h

The includes in the statismo sources and when the library are used in other project (THIS IS A MAJOR ISSUE FOR USERS OF THE LIBRARIES) uses then includes like:
new: #include 
old: #include

The statismoConfig.cmake sets then only one inlcude directory, the INSTALL_PREFIX/include directory which contains statismo/modules/headers.h

The CMake process needs only minor modifications already done in this pull request.

To summarize:

The pull request will provide much clearer includes.
The pull request will prevent possible header obfuscation.
Users of statismo need to adapt their includes ( → )
excellent idea to introduce such a variable