add validator checkbox_group, plus utility for numeric range

**checkbox_group**
added validator to check a group of checkboxes, to ensure required number of checkboxes were checked, using range (1-2), min1, or max2, etc.

**lang**
added prefix & suffixes for checkbox_group
renamed prefixes & suffixes used for length validation

**numericRangeCheck** 
added utility function to evaluate value against allowed parameter. abstracted internal check in validate_length to use this util function instead of repeating functionality

**validate_length**
edited function to use new utility function numericRangeCheck