Qtreemesh

- Added the method `adjust_mesh_for_FEM` to generate FEM-compatible mesh from the QuadTreeMesh

- Added a method 'vtk_export()' to create an unstructured grid vtk file from mesh.
- Resolved a bug related to adding midpoints to edges.

Added
- Start using a Changelog.
- Added docstrings for all classes, methods and functions.

Changed
- Modified 'draw()' method to remove axes from the figure and make it square.

Removed
- Unnecessary attributes of QTree class ('element_numbers','element_nodes' and 'element_type').