Continuing work started with PR #4016, PR #4026, and PR #4060. Improve documentation within program by making explicit what types of values are allowed for variables described as "mixed". In order to avoid broken functionality, this is done mainly through doc-blocks. This will get us closer to Phpstan Level 9, but many changes will be needed before we can consider that.
After this PR, 231 changes remain, all in src/Calculation.
Till now we have used a static variable/getter/setter to decide what type of result should be returned when a formula is evaluated and an array is the result. This is messy; it would be much better to use an instance variable instead. We cannot eliminate `setArrayReturnType` and `getArrayReturnType` because that would be a BC break. I am considering whether they should be deprecated. In the meantime, I have added a new instance property `instanceArrayReturnType` with getter and setter methods. The property is initially null, and, if it remains so when needed, the static property will be used instead. However, if it is set, its value will be used.
Owen Leibman
oleibman