join now works for nested cellstrs as well

This fixes https://github.com/nschloe/matlab2tikz/issues/229, https://github.com/nschloe/matlab2tikz/issues/246, https://github.com/nschloe/matlab2tikz/issues/234 (at least the original issue reported).

What happens in my branch, is that the type check is not performed  in advance anymore. The code accepts any cell array, as long as each item is either a char or a numeric or a cell array that conforms to those same (recursive) requirements. When a non-conformance is detected, the original error is thrown.

In case of a nested `cell`, `join` is called again with the same delimiter (maybe it makes more sense to have another one in different cases).

I'm thinking about the e.g. TikZ options (key value pairs) that are sometimes stored in cell, but where the delimiter between keys and values is `=` and between options is `,`. I, however, haven't looked more into that part of the code.