`.pxt`, `.pxp`, and `.ibw` files can now be opened with xarray methods such as `xr.open_dataset` and `xr.open_dataarray`. See the updated user guide for more information.

Calling `erlab.io.load_wave` and `erlab.io.load_experiment` is deprecated. When writing new code, use `xarray.load_dataarray` and `xarray.load_dataset` instead.

Whenever the `identify()` method failed to find any files, subclasses had to explicitly raise `FileNotFoundError`. This resulted in a lot of boilerplate code and ambiguous error messages. Now, all subclasses can just return `None` in `identify()` when no files are found. The appropriate error is automatically raised.

Memmapping seemed to interfere loading multiple files when called through ipywidgets.

The previous behavior allowed passing `f_001.pxt` to load `f_001_S001.pxt`, `f_001_S002.pxt`... but this was confusing since there is no file named `f_001.pxt`. This commit disallows such input.

Validation checks will not warn about missing detector angle, allowing XPS measurements.

…loaders

Combining before renaming coords should be more straightforward

A new attribute named `formatters` and a new method `get_formatted_attr_or_coord` has been added to loaders. This allows custom per-attribute pretty-printing behavior.

…entry