* add an extra parameter to lyxsweave.py that tells the document directory

  * add an extra parameter to lyxsweave.py that tells the document directory
and use it to be able to read data files in there (bug #6623). It is probably possible to use sweave hooks instead of the module preamble. Moreover, I want to investigate the use of prefix.string argument of sweave to make sure that the output files end up in the tmp dir.
  * be more subtle in the way to change default encoding to handle the case where a data file that is read does not use the same encoding as the .rnw file.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@36850 a592a061-630c-0410-9148-cb99ea01b6c8