Add locals for eval

[DanPorter]

Version 0.8.1

Added locals and named expressions in eval commands.

hdfmap_class.py

• Added add_local(**kwargs) to add data accessed by eval
• Added add_named_expression(**kwargs) to add short-hand expressions
• now uses is_image to determine if datasets are image data

nexus.py

• updated title in plot
• updated self.generate_image_data_from_nxdetector() to use is_image()
• changed generate_image_data_from_nxdetector() to add {detector}_image_list when image_data is available.

eval_functions.py

• updated eval_hdf() for new inputs
• added is_image() test

test_hdfmap_class.py

• added additional tests for new functionality

Updated docs.

All tests complete