Revisit API queries with new structure in place

[jomey]

After #104 is complete, let's revisit the API queries and see if we can simplify joins and make more use of subquery expressions.

[aaarendt]

In working on #170 I thought through other potential user needs within the API and am documenting here for possible inclusion when we revisit the queries:

• include kwarg for searching on depth ranges (e.g. what are all the densities between 50 and 80 cm depth?)
• the queries on values being greater / less than result in OR rather than AND logic (is this right?). Do we need to construct a specific case for ranges, e.g. 30 <= value <= 50? Seems like a common need, e.g. what are all the densities between 750 and 800 kg / m3.