Missing dijstra's algorithm

The dijstra's algorithm to calculate the distance between vertex pair is not yet implemented, although the function already accepts that type algorithm. This will raise an error. The relevant line of code is [here](https://github.com/DiedrichsenLab/DCBC/blob/1296f7ca24a69111392a1d8a14019c4be6dcf75e/utilities.py#L143). @jdiedrichsen