In the end it turned out that the cause of the phantom values was
short-cutting in getvalue() when the contents of a cell were empty,
preventing the update of the internal cache of the value of the cell.
However, tracking this down (and getting the associated memory management
correct) necessitated implementing a debugging mode in which I could
dump the internal states of cells and print various other stuff to standard
output. It also involved understanding the meaning of various pointers in
the code, in the process of which I renamed some commonly used macros,
particularly the former SHEET(s,x,y,z) which was not returning a Sheet at
all but rather a pointer to a Cell. So this macro is now called CELL_AT. I
also replaced several very repeatedly used patterns of checking the validity
of locations and pointers with macros, now defined in sheet.h.
Therefore, unfortunately the (relatively small in the end) bugfix for this
major issue is entangled with numerous textual changes to the code made
in tracking it down.
Fixes#18.
Closes#19.
