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>

.gitignore

@@ -1,3 +1,4 @@
 /flamegraph.svg
 /target
 Cargo.lock
+/tarpaulin-report.html