Added tests for spreadsheet scanner which hightlighted that

DECIMAL_NUMBER is not actually supported :-) 2 test failures for this.
Not looking good for this - its hard to tell a number from a decimal
number with poi.
This flushed out the header row check we'd already found and an issue
where it wasn't picking up the last row.
Added source folders for src/test/java and resources - I haven't added
the classpath - so you'll need to add them to your classpath manually.
There were a couple of personal overrides for me that I didn't want to
commit.
I added JUnit to the gradle dependencies and ran gradle check which
worked.
Not sure my code style really matches yours - sorry if its a bit off.
Hmm couldn't sleep and this was just bugging me to finish it off...