removed external projects: moving away from 'git subtree'
Resurrecting and updating according to progress of ViennaMini

- removed external/ folder: will use CMake to automatically
download dependent extenral projects -> no upstream for these
projects supported

- the framework compiles, ViennaMini module is deactivated for now
Added ViennaStar projects via Path variables+new device class

ViennaStar: using similar approach as with ViennaMini,
via cmake option we let the user choose between using
local viennaclones or downloading the projects via
cmake's 'externalproject_add' feature (the latter is
not available at the moment)

device class: is a minimized ViennaMini device class
-> so the mesh type is not parameterized into the
class via templates, allows easier handling
removed 'common.hpp' -> using 'forwards.hpp' now
using CMake's ExternalProject_Add to auto download/build external proejcts

also deactivated internals of viennamini module to get it to build first
updated readme.md to reflect build changes