[feature request] Add pseudo determinant

Related to #78

The [pseudo-determinant](https://en.wikipedia.org/wiki/Pseudo-determinant) is what one would like as the determinant of the pseudo-inverse, i.e., filter the extra zeros.

## Request

Add `pseudo_determinant` to `nalgebra::base::Matrix`.

## Motivation

When fitting a gaussian model with maximum likelihood to data, the covariance matrix may have zeros because there is little variation in the training data. Then, the likelihood needs to be estimated using the pseudo-inverse and pseudo-determinant.