stl_stats are newly only accessed by TriangleMesh::stats(),
most of the direct access to TriangleMesh::stl is gone with the exception
of parsing input files (3MF, AMF, obj).
Reworked the FFF testing framework & ConfigBase::set_deserialize()
for more compact tests: set_deserialize() now accepts list
of key / value pairs.
Fixed an incorrect assert in LayerRegion.
DynamicPrintConfig::full_print_config()
new cheaper constructors of DynamicConfig / DynamicPrintConfig from ConfigBase
Unit tests: ported test_model from upstream Slic3r, thanks @lordofhyphens
Unit tests refactored to use less autos and initializer lists for readibility,
DynamicPrintConfig is handled by value, not by shared pointer.
