Use of classmethod for _datamodels_open limits extensibility of Step

[braingram]

During debugging memory usage with @tapastro we found the implementation of _datamodels_open as a class method to be both limiting and lead to confusion.

For example, it would have been useful to change _datamodels_open to only call datamodels.open when:

• the input was not a DataModel
• or the step was not part of a pipeline

However the second condition is impossible as _datamodels_open is a class method.