new feature: Decimal Precision

[sandro furieri]

A further global connection setting is now available:

1) SetDecimalPrecision ( integer )
   when a decimal precision has been explicitly set then ST_AsText()
   will honor the required precision for all coordinate values.
   by setting a *negative* value the default precision (six decimal 
   digits) will be restored.
  
2) GetDecimalPrecision ( void )
   will return the current setting
  
Important notice: spatialite_gui too will honor this setting; all
floating point (DOUBLE) values will be displayed on the screen
accordingly to the currently set precision.

Please note: the traditional behaviour adopted by earlier versions
will always be implicitly assumed by default.
Anyway users wishing to adopt a different precision just have to
call SetDecimalPrecision() immediately after establishing a new
connection.

bye SAndro