Add option to sort categories alphabetically

Categories are now sorted alphabetically in the graphical user interface
for all the drop-down menus.

Fixes #91.
Order is preserved when concatenating headers

preprocess_mapping_file now preserves the order when column
concatenation is specified. If the columns Treatment and DOB are
intended to be concatenated, different results will be generated if the
user specifies 'DOB&&Treatment' and 'Treatment&&DOB'.

Fixes #93
Merge pull request #97 from ElDeveloper/issue_91

Add option to sort categories alphabetically
Merge pull request #98 from ElDeveloper/issue_93

Order is preserved when concatenating headers