Pyslvs

+ Add "set attribute" methods for expression classes.
+ Add "duplicate" method for graph class.
+ Some corrections of stub files.
+ Adjust Cython "cimport" to related import instead of direct import.
+ Change Sketch Solve solver function to a class.

+ Periodic update.

+ Add "Six bar linkage mechanism" configuration.
+ Speed up performance of Cartesian product function.
+ Nested do loop method for contracted graph enumeration.
+ Speed up performance of searching contracted graphs.
+ Function renaming of "link_synthesis", "contracted_link_synthesis" and "conventional_graph".
+ Allow empty links in mechanism expression grammar.
+ Changed into standard PyPA module.

+ More methods of graph class.
+ Support multi-graph for "Planar check" function.
+ Add contracted graph enumeration algorithm:
+ Speed up performance for searching between link assortment.

+ String `__version__` provided.
+ Triangulation data type changed for performance improvement.
+ Change "print" function as "logger.debug".
+ Embed the "Coordinate" object in return value to improve performance.
+ Use "is" instead of "==" in type comparison.
+ Enable C division as default for all Cython sources.

+ Change the function name of triangle formulas.
+ Remove the external color format of the VPoint color attribute.
+ Remove the "four bar constraint" option of planar linkage class.
+ Grammar enhancement.
+ Module "pmks" renamed to "expression".
+ Module "planarlinkage" renamed to "planar_linkage".
+ Adjust limitation option of algorithm to "max_gen", "min_fit" and "max_time".
+ Add "cdef" static method for joint creation.
+ Change "links" argument of VPoint constructor into string iterable object.
+ Using normal exception instead of "RuntimeError".
+ Supported specified link length option.
+ Cancel the copy step of submodule pxd file.
+ Enhancement of mechanism grammar.
+ Add "-j" option to speed up compile time.
+ Many performance improvements.
+ Remove the definition of inner scope.
+ Fix the memory leak of BFGS solver.
+ Reformat BFGS cython wrapper for more readability.