Validate that inputted number is a float and round

We limit the precision of the input box to 1 decimal place.

Note: Factor out the magic number (1) with in an idiomatic Javascript
way.
Fire off an AJAX request when the state changes

When a new value is entered into the input box, fire off an AJAX request
to get the tide windows for given location & tide level.

Note that it won't currently abort previous AJAX requests.