Add the ability to delay download of files with DataManager.require

[Cadair]

Describe the feature

Sometimes knowledge of what files are required by a function is conditional on the arguments to that function. It would therefore be useful if DataManager.require could optionally not download the data, that step being done in DataManager.get instead.

This would prevent the need for workarounds like this:

https://github.com/HinodeXRT/xrtpy/blob/660a539f3d6c7ee1e4596e86d4fdbbed5d74871e/xrtpy/image_correction/remove_lightleak.py#L213-L226