Adding attributes opened and filtered

[aleip]

Functions opendata(), filteropen() and filtermultiple() should set attributes to the data table in case it is saved for later inspection. Attributes should contain:

• name of files opened (thus an argument should be given to opendata() to indicate number of file that is opened)
• filtering done (indicating which dimensions were filtered with what)
• data of processing
• possible further information drawn from the set META present in the gdx files