make types mandatory in spec
make types mandatory in implementation
make types mandatory in test suite
better error messages for table index type problems
updated blt files to provide type data

finishes #92
Display error message if adding part fails

also more robust against undefined parameters.

fixes #90
removed unused cruft from utility script
added tasks subcommand
regenerated html
fixed typos in hex.blt
better error messages for iges generation
added iges quickstart
check for valid table index defaults

and test case
wrote spec for different drawing types
testcases for drawings
implementation for drawings
adapt backends
change drawing types to drawing-dimensions
connector drawings helper backend

This backend automatically generates scad files for the missing
connector drawings.
added connector drawings for all classes with connectors