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>

examples/planetlab.rs

@@ -11,7 +11,7 @@
 type FloatType = f32;
 
 /// `FloatType` is a type alias for either `f32` or `f64` depending on cargo features
-#[cfg(feature = "f64")]
+#[cfg(not(feature = "f32"))]
 type FloatType = f64;
 
 use std::{