Description

Actually the next-prev buttons are always enabled, if the user click on it and there are no step anymore, nothing happens. The button instead should be disabled at the beginning and at the end of data ( if the user selected a guide layer).

Acceptance Criteria

If a guide layer is selected, the next-prev button should be disabled if there are no more steps:

1. During normal use, next and previous data should be pre-loaded to enable next-prev buttons
2. During the animation it should check if next or previous steps are available in animation buffer to enable-disable buttons

Alternative solution

Disabling buttons during animation is almost trivial.
The things is more complicated in normal usage. Anytime the user selects a time, the application should do 2 ajax calls to find out if there are steps after and before the current time. To avoid this unnecessary requests, the application may disable the button after the button has been clicked, if no response was received. In case we adopt this solution point 1 of acceptance criteria becomes :

1. During normal use, if the user clicks on next, prev button and the there is no next or previous value to move to, the button disables.