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>

Cargo.toml

@@ -16,9 +16,8 @@ exclude = [
 ]
 
 [features]
-default = ["f32"]
+default = []
 f32 = []
-f64 = []
 
 [profile.bench]
 debug = true