mosaic_nrrd: multiple bugs with grid mode

[cpoczatek]

• Wrong raster value. On the test data fluid mode correctly gives the raster as "160,160" (that's microns) while grid mode gives "79998" (that's nanometers). Also I'm not remembering the rationale for changing units...
• The tile_positions field is missing.
• Doesn't delete temp .normalized files
• Broken-ish in ruby v1.8, makes a valid (ie openable) nrrd but the header is different than in v1.9 and has incorrect raster value.