Add `Filenames` base class for subclassing by readers; convert level 1&2 readers to use it

[claytharrison]

This PR adds a base class ascat.file_handling.Filenames which handles operations on filenames that are passed to it. It assumes child classes implement the needed methods out of: _read (for reading a single file), _write (for writing data to a single file), and _merge (for merging a list of data objects into a single one). It then handles the reading, writing, and merging of >=1 files using the subclass methods.

The PR also converts all the level 1 and 2 readers to Filenames subclasses along these lines. In fact, they are subclasses of a more general ascat.read_native.AscatFile subclass, which overrides the Filenames read method with some extra logic for spatio-temporal subsetting.

Some things I'm not sure on yet -

• Should subsetting be done on each file individually before merging or on the entire merged dataset after its read? I feel that the former makes more sense in terms of memory overhead but perhaps incurs unreasonable computational cost.

  • EDIT: Decided to do after read for now. May come back to this later.

• I had to add a read_generic init argument to the AscatFile class, which determines the default value for generic in the read method of an AscatFile object if read is not passed a value for generic when called. This is to reproduce the behavior of the old AscatL1bFile and AscatL2File classes, which instantiated reader classes and always used their read methods with generic=True by default, whereas the reader classes themselves had a default of generic=False. Perhaps there is a better solution though?

  • EDIT: Solved this by creating an, e.g., AscatL1bEpsFileGeneric class for every product, which simply redefines the _read method with generic=True as a default argument. These are the classes that AscatL1bFile and AscatL2File use to instantiate readers.

[claytharrison]

This also includes a few fixes for nan masking in the native readers that I caught while working on them.