Add raw data for testing

By "add" I mean "add a .gitignore entry so I don't upload a bunch of raw
data about my flat's heat meter & somehow dox myself" but still.

Nice to have testing data

Author: Lexicality

.gitignore

@@ -4,3 +4,6 @@
 
 # These are backup files generated by rustfmt
 **/*.rs.bk
+
+# Raw testing data from my meter
+raw_test_data/