SMD_matrix improvements

[Deleetdk]

Should add some things:

1. Option to get CIs on the values like cor_matrix. Standard and bootstrapped (when using non-parametric).
2. Why is this called _matrix and gives matrix (2x2) output when it only takes in one pair of variables? Because the group variable may have more than 2 levels. However, in the case of only 2 levels, it makes sense to simplify the output to a single value. Toggleable by argument otherwise the function would be unpredictably output type inconsistent.
3. Option to get the CIs in separate matrices for easy computational re-use.
4. Throw error when given length 1 input. Right now this just results in no errors and NA output. Confusing!