Fix NORDIF read. sig/nav size params overwrite + start io-tests

[hakonanes]

This PR addresses #67 and adds tests to the NORDIF reader and writer.

WIP since I'm in the process of writing the tests. I use HyperSpy's tests for the Blockfile format as inspiration (https://github.com/hyperspy/hyperspy/blob/RELEASE_next_minor/hyperspy/tests/io/test_blockfile.py).

Tests left:

• Loading testing for data shape and axes_manager, default input
• Loading testing input parameters pattern_size and scan_size
• Loading testing input parameters setting_file
• Load and save cycle (checking data shape and metadata/axes manager)
  - [ ] Load and save checking changing of axes manager All data info except intensities are read from the same settings textfile.
• Different loading modes (to memory, inplace, read only and lazy)
• Write signal from numpy arrays to NORDIF format
• Write 3D signal to NORDIF format (one navigation axis)
• Write 2D signal to NORDIF format (one pattern)
• Padding of missing patterns (when input scan_size or the scan_size read from Setting.txt is larger than file size)

Signed-off-by: Håkon Wiik Ånes hwaanes@gmail.com