Save results with pickle file becomes slow in case of a large number of time steps

[nicolasdlss]

When the number of time steps becomes large, e.g. 10 000, the saving to a pickle file becomes slow and takes a lot of memory:
Current approach

• CoCoNuT stores data in memory with among others numpy array with displacement, pressure and traction for every time step
• CoCoNuT writes data to pickle file (binary) based on save_results parameter

For large number of time steps:

• Large amount of memory required
• Writing of pickle file takes a long time
• Appending to large numpy array/lists takes a long time

[nicolasdlss]

Additionally, the parameter save_results can be changed to write_results to better reflect its function