Mrcfile

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* Fix incompatibility with numpy 1.16 by reading files into mutable bytearrays
instead of immutable bytes objects. Note this will cause slower reading of
bzip2-compressed files on Python 2 because Python 2's bz2 module does not
support reading directly into a bytearray and therefore the data must be
copied.
* Check that the file is large enough when reading the extended header.
(Previously, the size was only checked for reading the data array.)

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* Add automated tests for numpy 1.15 and Python 3.7
* Set maximum numpy version to 1.15 to avoid known incompatibility with 1.16

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* Add open_async() function for background file loading
* Add new_mmap() function for fast creation of large files
* Speed up mrcfile-header command line tool by reading headers only, not data
* Allow opening of files with incorrect machine stamps in permissive mode
* Fix bug in validation of exttyp field in Python 3 (thanks to Holger Kohr)
* Fix bug in opening files with misleading exttyp information
* Update documentation to point to Python 3.6 instead of 2.7, and fix all
cross-references
* Other documentation improvements

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* Add command-line tools ('mrcfile-validate' and 'mrcfile-header') for easy MRC
file validation and header viewing

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* Fix bugs due to changed behaviour in numpy 1.14
* Add automated tests for numpy 1.13 and 1.14

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* Add support for FEI-format extended headers
* Documentation improvements