Reintroduce esofile processing

[voightp]

unlike the .sql, we should instantiate the Reader and call the object get_results method (to avoid re-reading the .eso file for each call)

[Tokarzewski]

Query multiple times without re-reading the file

eso_file = DBEsoFile.from_path(eso_path)  # Parse file once into memory

results_eso_1 = eso_file.get_results(
    variables=[Variable(None, None, "C")],
    frequency=H
)

results_eso_2 = **eso_file**.get_results(
    variables=[Variable(None, None, "Pa")],
    frequency=H
)

[voightp]

Done in #14