in get_multilevel_element, allow user to specify an element that's not a direct child element of a level aka a specific xpath

[atn38]

relates to #9

so I've found a way to write a generic function that grabs elements from multiple levels (dataset, entity, and attribute) of an EML document. you specify the element name and the function that parses it, then the generic function checks for the element name in the names at each level.

however, how can one specify an element that's not a direct child element of a level? for example, coverage is a possible child element of entity, but one may want to get coverage/geographicCoverage instead.