Issue #134: Add option to options file for displaying absolute value,…

[AndrewLister-STFC]

… relative, or both

Description of Work

Add an option 'comparison_mode' to the options file. This will change the value reported in the results table.
If comparison_mode is:

• 'abs' : it will report the absolute value from the run (note: the colour will still be relative to the other results)
• 'rel' : it will function as it did before this change
• 'both' : it will display the absolute value followed by the relative in parentheses. e.g. 0.001274 (1)

Fixes #134

Testing Instructions

1. Set 'comparison_mode' in options file
2. Run example scripts
3. Check output is as expected

[wathen]

@AndrewLister-STFC can you change the width of the first column so that it is about the length of the name of the longest data set?