Add support for joins

[lindholc]

Adds support for making joins using the read_data function.

Example (also in the README):

df = read_data(
    "https://example.org/latis/dap2",
    ["dataset1", "dataset2"],
    "yyyy-mm-dd",
    "yyyy-mm-dd"
)

We will make a request to the join service if 1) the LaTiS instance being queried is assumed to be a LaTiS 3 instance (base URL ends with /dap2) and 2) read_data is given a list or tuple of dataset identifiers. If the LaTiS instance is assumed to be a LaTiS 2 instance, we'll raise an exception.

This guesses the join service URL by replacing /dap2 with /join. Queries are passed to the endpoint like a /dap2 request and the results of /dap2 and /join queries are handled exactly the same.