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- writes out correct number of blendmethods
- adds checks for object-level property on writing
- sets an object level property in SetAllTriangleProperties (if object-level property has not been set before).

* Allow clearing of triangle properties
* Prepare object-level properties
* Implement object-level properties

According to:
- https://github.com/3MFConsortium/spec_core/blob/master/3MF%20Core%20Specification.md#413-vertices
- https://github.com/3MFConsortium/spec_core/blob/master/3MF%20Core%20Specification.md#414-triangles
- https://github.com/3MFConsortium/spec_beamlattice/blob/master/3MF%20Beam%20Lattice%20Extension.md#111-beams

A raw pointer to an object was converted into a shared pointer and
stored as member of an object instance. Eventually, the object was
deleted twice. This behaviour has been resolved.

+ remove warnings regarding #pragma(push)

It appears that
1. travis sometimes uses other than the default linux distribution (trusty, 14.04) specified here:
    https://docs.travis-ci.com/user/reference/overview/#linux
2. the default setup of mingw on xenial (16.04) has trouble with std::mutex, which is why lib3mf builds fail on this machine.

Thus, this commit enforces trusty as distribution for the mandatory mingw-builds.

Reading
- relative progress is now based on amount of data read
- progress identifier is still defined via which element is currently being read

Writing
- relative progress is now based on number of resources and attachments
- progress identifier is still defined via which element is currently being written

Performance:
- replaced inefficient data structures (list or vector + linear search for elements)
  by more apt datastructures (map (unordered_map, multimap) + lookup) in
  a couple of occurences

This major commit prepares v2 of lib3mf
- adds the Automatic Component toolkit
  - this removes the COM interface
  - autogenerates language bindings for multiple languages
- reworks the way in which properties are handled
- implementes multiproperties and compositematerials
- adds basic (and automatic) documentation using sphinx / https://readthedocs.org/
- improves/changes lib3mf in multiple other minor ways
- ... but has several open issues left

* add CImportStream_Shared_Memory and CImportStream_Unique_Memory

* Fix bad_alloc -Wcatch-value from GCC 8

* use tab instead of 4 spaces

* increase patch version to 1.8.2

Specify next release to be v1.8.1