Asammdf

Fixes
* handle empty channel groups
* do not alter comments (XML content) created by other tools (for example CANape display information)
* disk IO improvements

Fixes
* ensure maximum Matlab variable name for .mat export
* fix handling of CAN bus logging with ErrorFrame and RemoteFrame (issue 108)

Changes
* improve robustness of XML comments parsing
* include offset information for concatenated time basis
* performance improvements
* improve performance of string channels
* optimize channel reading for channels without attachments or default X axis
* improve MDF methods by avoiding copying the master channels


Fixes
* fix typeerror when using numexor evaluate
* fixes in Signal cut

Fixes
* fix big endian handling of channels that have bit offset or non standard size
* fix Signal _cut_ method
* fix MDF _concatenate_ method
* fix benchmark script for new internal package structure
* fixes in plotting
* correctly use np.append when extending Signals

Changes
* add _include_ends_ argument to Signal _cut_ method
* remove _cantools_ and _matplotlib_ from requirements
* add _record_offset_ argument to MDF _select_ and MDF4 _get_ methods
* MDF4 compressed blocks speed optimizations

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