fixed bug in get_name
importing BOLTS outside FreeCAD

before the module would try to create and attach a widget, which
prevented its use as a plain python module without FreeCAD.
addded specification for common parameters
implementation of common parameter combinations
static checks and statistics for common parameters
handle failing backend imports gracefully
fixed problems with backend assumptions

some backends assumed others to be present. Checked for that and tried
to handle these cases as good as possible.
step backend

not really polished, but working
add step backend to repo and use good output path

Using the a version dependent string as an additional directory level
will make it easier to keep several versions of the distributions in
parallel.
removed all non-essential parts
reworked and simplified blt file parser
fixed typos introduced during refactoring
added generator for iterating over all class ids

this is a common usecase, so we provide a convenient way to do it. The
generator returns one class for every class id. As many different
class instances can exist for every id, one should be careful only to
use class properties when using this iterator, not e.g. properties of
one of the standards of this class.
moved schema checking back into a function

it is more flexible this way
cleaned up openscad module
cleaned up freecad
documented some changes done during refactoring
cleaned up tests
readded static checker script
fixed a few pylint errors