Documentation updates
Use divide method instead of / when scaling flat

Closes #92
Change tests of flat_correct to use flats whose value are not all the same

When the flat has values that are all the same, the flat_correct ends up just dividing by 1.
Change flat_correct to use results of arithmetic methods

The methods return a new CCDData, leaving the original unchanged.
Fix reference to data in test of flat_correct
Avoid copying flat and never modify input flat
Merge pull request #116 from mwcraig/fix-divide

Use divide method (and fix a couple errors that uncovered)