added the estimate compression ratio script

estimate_compression_ratio.sql
  uses the statistics data to estimate the compression ratio of OLTP / HCC compressed table partitions
  for this it estimates the uncompressed table size by multiplying the average row length with the number of rows and devide the result by the block size
  to get the current (compressed) table size, the number of blocks (as stored by the cbo) is multiplied by the block size, rather then getting it from dba_segments.
  this to avoid errors when the cbo statistics are stale (as the calculation for the compressed and uncompressed size are based upon the same data)