Enhance to handle xml with missing 'type' attributes

Recent example data shows xml where the 'type' attribute is not present on every element in the xml tree, currently resqpy relies on this type attribute. Enhancements are required to rely instead on the element tag, rather than the type attribute, to read these datasets.

Start by locating the resqpy XML dataset reader and tracing where each element's 'type' attribute is required. Compare that path with the element tags in the recent example data. Done means datasets whose XML elements omit 'type' attributes can still be read using their tags.

Written by the indexing model from the issue text.