Computes statistics for numeric and string columns

# Computes statistics for numeric and string columns

df.describe(['age']).show()
# +-------+------------------+
# |summary|               age|
# +-------+------------------+
# |  count|                 2|
# |   mean|               3.5|
# | stddev|2.1213203435596424|
# |    min|                 2|
# |    max|                 5|
# +-------+------------------+
df.describe().show()
# +-------+------------------+-----+
# |summary|               age| name|
# +-------+------------------+-----+
# |  count|                 2|    2|
# |   mean|               3.5| null|
# | stddev|2.1213203435596424| null|
# |    min|                 2|Alice|
# |    max|                 5|  Bob|
# +-------+------------------+-----+