Mrcfile

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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

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* Add automated tests for Python 3.4 and 3.6
* Documentation improvements