Create comparing decimals 1 exercise
Create comparing decimals 2 exercise
Add visual model to hints
Improve visual model explanation
Minor tweaks
Rewrite hints for exercise 2
Merge branch 'comp-dec1' of https://github.com/petercollingridge/khan-exercises
Fix answer compuation in comparing_decimals_1

Test plan: Answer a few questions in comparing_decimals_1 locally

Auditors: peter