Utility function to convert an ushort array into a double

Converts four ushorts into a IEEE 64 floating point value.
An "extension" to its sister function GetSingle(). Needed to convert responses from S7-1200's LReal data type registers.
Had a hard time in naming the arguments so I ended up with b3 (byte 3), b2 (byte 2), etc., from highest-order to lowest-order value.