added freecad library as submodule
added remark about submodules
draft of a base function for fcstd bases
Automatic freecad base function discovery

fixes #25
Merge branch 'master' into fcstd_base
fixed typo
cleaned up blt_parser
removed generated file
moved base specifications to common
simplified error handling a lot

errors now derive from a common base class which allowed for a
consistent and efficient way to fill in full tracing information, which
makes exceptions much more useful.

Also improved the test suite, by adding some error related tests.
first draft for base file layout for different backends
better traces for parsing errors