Various enhancements and bug fixes

• Linked items within a file can now have different names from their target. This restriction was lifted a few years ago.
• Added a compression filter setting, NX_COMPRESSION, which defaults to 'gzip'. This can be set using the nxsetcompression function, e.g., nxsetcompression('lzf'). It can also be set to ‘None’.
• Added a unicode encoding setting, NX_ENCODING, which defaults to 'utf-8'. This can be set using the nxsetencoding function, e.g., nxsetencoding('latin1').
• The nexusformat package now uses variable length strings for storing all text fields using utf-8 (the h5py default), but fixed length strings in existing files could have a variety of encodings. If there is a decoding error when reading a string, nexusformat will see if 'latin1', the most common alternative, works. However, there is no guarantee that the correct encoding will be selected.
• The nxtree function has been renamed nxdir, to prevent clashes with the NeXpy GUI.
• Improved handling of infinities when plotting.
• Improved copying of large datasets between files.
• Improved handling of singleton and reversed axes.
• Fixed a bug that caused a shift when slicing data whose axes are stored as bin boundaries rather than centers.
• Added a __nonzero__ function to NXfield. Note that this means that if statements involving the NXfields will return False if all items are 0 and True otherwise.