Ingest different DRS formats

This PR is a significant rewrite of how we handle DRS file paths to allow us to ingest paths of a few different known DRS formats.

This involved:

• creating a new crate to handle just loading the different DRS types
• changing ingestion to load the different types and then convert them to our metadata format
• fix some funky issues with data-dir not behaving how it should
  • I also made it optional so not having it means that all datasets are ingested

drs/src/lib.rs

 #![warn(missing_docs)]
 //! This module handles extracting metadata from various DRS path specifications.
+//!
+//! It presents functions which take in DRS paths and returns objects containing the path given (to facilitate opening
+//! the file later if needed) along with an object holding the metadata found in the path. Currently it does not provide
+//! functionality to open the files and extract any metadata from the file's attribute data.
+//!
+//! # Example:
+//! ```
+//! use camino::Utf8Path;
+//! use drs::cmip5::Cmip5;
+//!
+//! let path = concat!(
+//!         "cmip5/output1/NOAA-GFDL/GFDL-CM3/1pctCO2/fx/ocean/fx/r0i0p0/v20120227/gridspec/",
+//!         "gridspec_ocean_fx_GFDL-CM3_1pctCO2_r0i0p0.nc"
+//!     );
+//! let cmip5 = Cmip5::from_esgf_path(path.into()).unwrap();
+//! assert_eq!("NOAA-GFDL", cmip5.metadata.institute);
+//! ```
+//!
+//! This relies heavily on the [`nom`](https://crates.io/crates/nom) for parsing the paths which allows for this to be
+//! fast with minimal allocations hence the `metadata` objects will reference parts of `path`. Any comparisons which
+//! are case insensitive do allocate a temporary string as part of `to_lowercase`.
 
 pub mod cmip5;
 pub mod cmip6;