Add a Square type parameter

[mzgubic]

As discussed in #94, this package does not publicly distinguish between square and non-square BlockDiagonal matrices, and @nickrobinson251 convinced me that is the right thing to do.

However, it might be useful to distinguish between square and non-square BlockDiagonals internally for the following reasons:

• It prevents type instability when the return type depends on the "squarity" of the blocks, as in Adding a BlockDiagonal and Diagonal matrix fails for non-square matrices #103
• It makes the code more Julian by allowing us to dispatch rather than use an if statement