getErdsValues

 Extracts ERDS values of ERDS maps.

 This function returns the ERDS values within a specified time and frequency
 window from an already calculated map.

 Usage:
   values = extractErdsValues(r, time, freq);

 Input parameters:
   r    ... ERDS map structure
   time ... Time window (s)
   freq ... Frequency window (Hz)

 Output parameter:
   values ... Structure containing information about the ERDS map