The summary method displays standard information about a strata class object.
It displays a synopsis of the content in the strata object, including total number of layers,
thickness of the study section, and number of layers by lithology type. Once the stratigraphy
data are loaded into R, and validated as a strata valid object, the users are able
to explore stratigraphic information quantitatively (e.g., summarizing it by lithology type or grain size).