Apply fixes from StyleCI

[mechelon]

This pull request applies code style fixes from an analysis carried out by StyleCI.

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[what-the-diff]

• The AND and OR constants were changed from CONST to const.
• A test was added for a filter bag with the AND operator and two conditions, which checks that it creates an array of filters as expected (with one key-value pair per condition).
• Another test was added for a filter bag with the OR operator and three conditions, which also checks that it creates an array of filters as expected (with one key-value pair per condition). It then adds another nested compound query inside this first compound query using the same logic - checking each value in turn against what is returned by $filterBagQuery[Operators::OR][1] etc., where Operators::OR refers to 'or' in JSON format ('and' would be used if we wanted all 3 values at once)
• Finally there's a new exception thrown when nesting too many bags within other bags: "The maximum nesting level of compound filters must not exceed 2." This means you can't have more than 1 layer deep on your queries; e.g.: