test: get to 100% coverage, clean up cargo features

- eliminated `f64` feature because Cargo doesn’t understand mutually exclusive features. So, we default to `f64`, and leave the `f32` feature there if people want to change to that.
- integrated tarpaulin for coverage
- added tests to get to 100% coverage
- fixed NetworkCoordinate Div impl safety checks (now acts like Add and Sub)
- update README to explain cargo feature change
- cleanup build script with the new simpler features model

Signed-off-by: Stephen Waits <steve@waits.net>

README.md

@@ -112,11 +112,13 @@ usage examples.
 
 ### Cargo Features
 
-By default, the internal data structures and operations all use `f32`. If you want to use `f64`, you can enable it as a cargo feature:
+By default, the internal data structures and operations all use `f64`, which is
+slightly faster on modern architectures. If you want to use `f32` instead, you
+can enable it as a cargo feature:
 
 ```toml
 [dependencies]
-vivaldi-nc = { version = "0.5.2", features = ["f64"] }
+vivaldi-nc = { version = "(version)", features = ["f32"] }
 ```
 
 ### Examples