Add method to mask matrices to certain zones

[BenTaylor-TfN]

Add a method into the Pandas and Numpy utils that allows the cutting down of a matrix into a subset of it's data.

This is needed in the data requests work here: https://github.com/Transport-for-the-North/data_requests/issues/22

See the following code in NorMITs Demand for examples of how to do this:

• https://github.com/Transport-for-the-North/NorMITs-Demand/blob/12d99c0da03a1fbb9e95011c09a642df36e0f2b6/normits_demand/utils/pandas_utils.py#L679
• https://github.com/Transport-for-the-North/NorMITs-Demand/blob/12d99c0da03a1fbb9e95011c09a642df36e0f2b6/normits_demand/utils/pandas_utils.py#L761
• https://github.com/Transport-for-the-North/NorMITs-Demand/blob/12d99c0da03a1fbb9e95011c09a642df36e0f2b6/normits_demand/utils/pandas_utils.py#L783