Gdshelpers

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* Allow explicit definition of datatype by passing a tuple (layer, datatype) as layer
* Allow setting the width of LineStrings
* Angle of sub-cells considered in DLW-data and boundaries
* Fix GDSII-export: allow negative angles, correct rounding of spacing
* Allow specifying clearance features and exlusion features for convert_to_positive_resist

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* Added meep integration
* GDSII-export: Added support for LineStrings

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* Waveguide: added add_left_bend and add_right_bend to make code easier readable
* added alphanumeric_to_id as an inverse of id_to_alphanumeric
* allow to limit the numbers of workers for parallel export
* fixed oasis export

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* Grating coupler: make_traditional_coupler now allows to apodize the period of the grating
* Port: added with_width function to generate a copy of the Port with a certain width
* Increased precision in add_straight_segment by evaluating derivative
* Added add_route_straight_to_port to Waveguide
* Fixed evaluation of width-parameter in add_parametrized_path
* Stopped testing with Python 3.5, as it reached it's end-of-life and added a warning
* Deprecated gdsCAD, as it isn't compatible with Python 3
* Fixed cell.show

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* Added scale-parameter to save_image
* fixed .dxf-export in Cell
* Waveguide.add_parameterized_path now also supports an array as path_derivative
* fixed add_dlw_marker, origin can now also be a list

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* Removed \_\_future\_\_ imports and (object) in class definitions for Python 2
* create_holes_for_under_etching now allows ovals and rectangles
* add_route_single_circle_to_port now tapers the waveguide to match the width of the port
* Bugfixes