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.
SINGLE function can be used to return first value from a dynamic array result, or to return the value of the cell which matches the current row (VALUE error if not match) for a range. Excel allows you to specify an at-sign unary operator rather than SINGLE function; this PR does not permit that.
Add support for reading CSE array functions for Gnumeric.
Throw an exception if setValueExplicit Formula is invalid (not a string, or doesn't begin with equal sign. This is equivalent to what happens when setValueExplicit Numeric specifies a non-numeric value.
Added a number of tests from PR #2787.
Owen Leibman
oleibman