Asammdf

Fixes
* fix extract_mux (using muxer_values instead of muxer_values_)
* check if the invalidation bits length matches the samples length when creating new Signal objects
* fix value searching in the numeric window
* fix stacking of signals in the plot

Changes
* compute the time base when deleting signals from the Plot or when enabling/disabling signals
* hide cursor when the region is active
* allow locking one region end in the plot and controlling which region end is moved on click

Fixes
* fix extract_mux (using muxer_values insteaf of muxer_values_)

Fixes
* add lz4 requirement to setup.py

Standalone GUI builds
* [Linux](https://ci.appveyor.com/api/buildjobs/tm3mkyh3irfjhuf0/artifacts/dist%2Fasammdfgui_noconsole)
* [Windows](https://ci.appveyor.com/api/buildjobs/srb61q74bo728k2f/artifacts/dist%2Fasammdfgui_37-x64_noconsole.exe)

Fixes
* fixes 286, 287
* fix output file extension when scrambling a MDF v3 file
* fix CSV export
* set the correct channel group source for bus logging measurements
* set the Signal encoding attribute correctly
* use backslashreplace when encoding strings to latin-1 to avoid exceptions
* fix transfering mdf v4 TABX conversions to mdf v3
* save the display name when appending to mdf v3
* fix Signal.interp when the new timestamps has len 0
* fix attribute error in case of TAB convertsions when converting from MDF v3 to MDF v4
* fix malformed XML channel comment when saving a MDF v4 signal that has the display name set
* correctly handle extended CAN id's
* fix can extraction for OBD2 muxed signals
* correctly ignore the invalid CAN signal extracted from bus logging measurements




Changes
* refactoring of MDF methods to use the virtual/distributed channel groups
* use LZ4 to compress intermediate data in the MDF methods
* improve integer values validation when setting the target value in the Tabular window filters
* improve GUI speed when loading a file with a high channel count
* improve garbage collection
* add _use_display_name_ keyword argument to concatenate and stack methods
* add option to validate the signals returned by the MDF.select method
* support display files

Fixes
* fix attribute error in MF4 class

Fixes
* fixes 279, 281
* support channel arrays with conversions
* fix for look-up channel arrays with no samples
* fix MDF.select when it is called with record_count!=None
* fix for displaying channel information in GUi for MDF v3 measurements
* fix Tabular window in case of string channels
* fix extract_can_logging and extract_can_signal when the payload size is smaller than the message DLC found in the CAN database
* set the actual record count in the yielded data of _load_data
* fix functionality related to the "Filter" tab in the GUI

Changes
* added _iter_to_dataframe_ method
* added _\_\_lt\_\__ special method to the MDF class to allow sorting of measurement files based on the header timestamp
* add initial implementation for visualizing channel from multiple files in the same plot