snap pre modifications

[johntruckenbrodt]

This makes some modifications to the processing workflow so that the products generated during preprocessing are not modified in succeeding steps:

• snap.nrt_slice_num() is now called in snap.pre() instead of snap.grd_buffer()
• ancillary.datamask() is now called in snap.process(). In ard.format() it is still called but only to retrieve the file name of the previously created mask and not to actually create it

This way, the products are finalized once snap.pre() terminates and no other process needs to write-lock them for modification, which avoids conflicts in parallel processes.