Remove 'Layer' and switch to 'Variable' mechanic

Closes #14 (closed), #7

• simplified whole NetCDF interaction
• inputs/outputs are now given as a list of strings or Variable instances
• if a string, it will be converted to Variable by Variable(name)
• IO name now matches variable name by default (if it should be different, one can use Variable(name, io_name=other_name))
• time variable name is now determined automatically
• added ability to add user specified info kwargs with the Variable
  • specify input units
  • overwrite grid specs
  • add/overwrite meta data
• writer now allows for multiple grids in one file as long as their axis names are unique
• slicing is now possible with all dimensions (Layer.fixed was renamed to Variable.slices)