Asammdf

Fixes
* fixes issue 133
* fixes issue 132
* open files using mmap in read-only mode

Changes
* use period keyboard button to set curve dots in GUI

Fixes
* fix metadata method for v3 blocks

Fixes
* fix handling of channel attachments
* fix handling of CAN bus logging
* use physical values for value to text conversions when exporting to pandas dataframe

Changes
* added _--measurements_ cli argument to GUI to load a set of measurement at startup

Major changes
* minimum Python version is now 3.6
* _memory_ option was removed; the new implementation behaves like _memory='low'_ from the 4.7.11 release, but has better performance and resource usage
* graphical interface requires PyQt5
* new computed channels can be added to the plot in GUI
* improved performance of GUI (with large channel count and sample count)

Fixes
* fix ``local vcariable 'trucated_integers' referenced before assignement`` in _as_non_byte_sized_signed_int_