Make default cell class behavior consistent between implementations

[jessicasyu]

If you do not define a population class in the setup file when running the patch implementation, you get an error because a default cell class is not defined. With potts, if a population class is not defined, the series instance will use a DEFAULT_CELL_CLASS instead:

ARCADE/src/arcade/potts/sim/PottsSeries.java

Line 18 in 600b782

public static final String DEFAULT_CELL_CLASS = "stem";

We should make the behavior consistent between the two implementations. Options:

1. Update patch to support the DEFAULT_CELL_CLASS behavior (with the default class as tissue)
2. Require that class is defined in the setup file and remove the DEFAULT_CELL_CLASS from potts

I think (1) would be the preferred option, but I could see (2) making setup files more clear and would work if we have the appropriate error message.