Add a codecov.yml

The coverall build is stuck at 0% (0 of 0 relevant lines covered)
in some branches, but ok for others. The first warning was, there
is no codecov.yml file - so add one.

https://docs.codecov.io/docs/codecovyml-reference

codecov.yml

@@ -0,0 +1,21 @@
+codecov:
+  require_ci_to_pass: yes
+
+coverage:
+  precision: 2
+  round: nearest
+  range: "70...100"
+
+parsers:
+  gcov:
+    branch_detection:
+      conditional: yes
+      loop: yes
+      method: no
+      macro: no
+
+comment:
+  layout: "reach,diff,flags,files,footer"
+  behavior: default
+  require_changes: no
+