Provides a form element for a set of checkboxes.
Properties:
#options: An associative array whose keys are the values returned for each checkbox, and whose values are the labels next to each checkbox. The #options array cannot have a 0 key, as it would not be possible to discern checked and unchecked states.
Usage example:
$form['high_school']['tests_taken'] = array(
'#type' => 'checkboxes',
'#options' => array('SAT' => $this->t('SAT'), 'ACT' => $this->t('ACT')),
'#ti