Added user.clj to dev profile to be able to work nicely from the repl.
Changed lazy-sheet so that is returns the cells as logical defined as opposed
to the physical definition.

The old implementation would skip blank (undefined) cells which is quite
strange. There are now two modes for lazy-sheet :logical and :physical. The
:physical mode does the same as the old implementation in the case someone
needs it.

This commit breaks the signature of lazy-sheet as it now takes the cell-fn
and mode as map-style options.