add option to remove windows with poor data quality

[jasminerienecker]

This review adds the parameters data_availability_threshold (defaults to 0.0 to maintain currently functionality) to all models that inherit the BaseWindows class. This parameters allows us to discard windows where the percentage of good quality data points is below the threshold. The quality of a data point is determined by the corresponding value in the available_mask column.

This is a functionality I currently require as my dataset has many large gaps and I don't want to be training the model using these gaps.

I have added a test to the end of base_windows notebook.