Feature: code styling change of increment to (custom) variable

[DSCaskey]

Make code style changes of the term "increment" to "variable'

• Change xml element tag < increments > to < variables >
• Change xml element tag < increment > to < variable >
• Change the term "increment" to "variable" and where neccessary "custom variable" in class names, class method names, class variables, method arguments, and assorted comments.
• Change filenames to use "custom_variable" instead of "vincrement"