Memory caching for DM3/DM4 files and ncem2png command

[gonzalorodrigo]

This PR brings two new features to the library:

Memory caching for DM3/DM4 file: DM3/DM4 file parsing requires reads of random positions of the file. Such I/O pattern makes read operations slower, especially if the file is hosted in a network storage system. To ease this problem, this PR pre-loads the file content in memory and provides an API that wrap the file read operations and direct them the copy cached in memory.

Main Changes:

• fileDM class pre-loads the content of the source file in memory (if "on_memory" mode).
• fileDM class new wrapping methods: from_file (np.fromfile), tell (file.tell), and seek (file.seek) methods that read from a file or the copy memory.
• added unittest for io on DM files.

Results: x1000 speed up while executing on a NERSC super computing reading from a network parallel file system.

ncem2png: a new command-line command to extract 2D images (in PNG format) from SER, DM3, and DM4 files. Main features:

• It can process multiple files in a single call.
• For images with more than two dimensions it is possible to select the method to extract a 2D image from the data: What dimensions to fix and in which value.

[ercius]

The memorydm branch from @gonzalorodrigo was tested locally by myself and works great. Thanks for adding this feature to openNCEM!