File import / export for Dynamics and Process Tensors

[gefux]

The current method to import and export Dynamics and Process Tensors using pickle is not ideal.
I suggest to use the .hdf5 file format instead, this allows:

• portability to other languages (C++, Julia, Wolfram,... )
• to save memory in the exporting process (.pickle needs to copy the entire object before writing to disc)
• to only have the parts in memory that are actually needed at one point in time in a computation.

A (really) minor draw back: The types in the .description_dict property will need to be restricted to data types compatible with HDF5

[gefux]

While I am at it: I will also reorganise the ProcessTensor classes a bit and create a BaseProcessTensor class to allow many different inherited ProcessTensor classes (a "regular" version and a "saving memory version" version where the data is primarily on file on only the requested part of the ProcessTensor gets loaded into memory, etc.)

[gefux]

Pull request #15 did most of the job (documentation and new tests are still missing).
Because it contains several breaking changes, I've merged it into a version 0.2 development branch.

[gefux]

The new process tensors (including import/export to .hdf5 files) is now on the OQuPy-dev branch. To keep it simple I've decided against a separate file format for Dynamics objects. Saving dynamics should be done by simply saving the .times and the .states in a file format of your choice.

ToDo:

1. testing of process_tensor.py
2. docstrings in process_tensors.py
3. write examples for import & export of process tensors

[gefux]

As import and export of process tensors is implemented I close this issue and create a followup issue for testing and documentation: #103