Feature/18 add bi temporality references in data contract (#20)

[#18] Add bi-temporality references in data contract
Fix typos and style in README and in example.yaml
Add bi-temporality fields in example, readme and cue verification file
Add .gitignore
Fix indentation (now only spaces are there) in the cue file

Author: erond

.gitignore

@@ -0,0 +1,4 @@
+# Default ignored files
+/.idea/workspace.xml
+/.idea
+/.idea/*