* Implementation of MemoryFile.write
* Complete initial impl of MemoryFile and tests
* Adjustment in tests and read() for lack of bytes in Python 2.
* Relax file size assertion, skip tests for GDAL<2.0
* Write a __len__ backed by VSIGetMemFileBuffer()
* Remove leftover except clause
* Add a 'closed' attribute to MemoryFile.
And various code improvements suggested by @brendan-ward.
* Rename check() to exists() and remove entry from pxd
* Revert change to DatasetBase, use case handled in MemoryFile