Create yield curve, qlab library, and Cargo workspace

Created qlab library with root Cargo workspace with qlab-time, qlab-error, qlab-termstructure and qlab-instrument crates. Implemented basic date operations, Bond structures, and yield curve functionalities. Added related dependencies and tests to ensure functionality.

.gitignore

@@ -3,12 +3,13 @@
 debug/
 target/
 
-# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
-# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
-
 # These are backup files generated by rustfmt
 **/*.rs.bk
 
 # MSVC Windows builds of rustc generate these, which store debugging information
 *.pdb
+
+
+# Added by cargo
+
+/target