Implement equations mode in parseWilkinsonFormula.

[AvanishSalunke]

documentation and demos still remaining, working on it. (Keeping this PR on draft).

1. Only table input is supported.
2. Equations mode is implemented.
3. nested factors is also supported which was partially implemented, equations mode is processed differently than other modes.

Closes #390

[pr0m1th3as]

Please, use single quotes when specifying cell arrays of character vectors, as in {'a', 's'} instead of {"a", "b"}. Sooner or later Octave will change the semantics to natively support constructing string arrays with double quotes as MATLAB does, and then {"a","b"} will no longer be a cell array of character vectors but a cell array containing a string scalar in each cell element.

For the aforementioned reason, use cell array of character vectors to construct a string array instead of a char array using double quotes. There are a few cases in the tests I noticed.