Asammdf

Changes
* refactoring of code related to GUI
* emphasize the use of _common_timebase_ argument of _append_ method (documentation update)

Fixes
* fix invalidation bits handling in Signal extend method

Changes
* changed _Signal extend_ method

Fixes
* fix for issue 103
* fix documentation for _extend_ method
* fix _concatenate_ in case of _sync=False_

Changes
* expose _oned_as_ argument (see [savemat](https://docs.scipy.org/doc/scipy/reference/generated/scipy.io.savemat.html)) in case of _mat_ export

Fixes
* fix issue 102
* use 4MB fragment size in GUI

Changes
* improve performance of GUI

Changes
* added _get_group_ method to MDF class
* improve the memory efficiency of concatenate method

Fixes
* fix wrong header comment for MDF3 saved using memory='minimum'
* fix _stack_ method

Changes
* _Signal plot_ method tries to use pyqtgraph before matplotlib

* the same keyboard shortcuts as in the GUI can be used

* string channels and value-to-text channels can now be plotted
* use write fragments for signal data referenced by channels
*

Fixes
* fix typo in _scramble_ method
* set a maximum limit of 4MB for write fragments to ensure compatibility with CANape
* fix in _select_ for channel groups with 0 cycles and memory='full'
* fixes in value range to text conversions
* fixes in GUI in case of displaying a single empty channel
* use correct icons for left and right keyboard shortcuts