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/cmip5.rs

@@ -204,8 +204,8 @@ impl ToString for Frequency {
     fn to_string(&self) -> String {
         use Frequency::*;
         let s = match self {
-            Year => "year",
-            Month => "month",
+            Year => "yr",
+            Month => "mon",
             Day => "day",
             Hour6 => "6hr",
             Hour3 => "3hr",