Temporarily disabling the protrusion plugin

When using the protrusion plugin with a cell that does not perfectly fit the other constraints, such as the volume and surface constraints, all lattice sites immediately get high actin levels because the cell is changing it's shape to accommodate the other constraints. To prevent this, I tried to turn of the protrusion constraint temporarily, which turned out to be harder than I thought.

• Setting the strength of the plugin to 0 disables the plugin from affecting the Hamiltonian, but the actin field still develops.
• Setting the strength of the plugin to 0 and setting the maximum actin level to 0 resulted in the frozen cell. This happens because the plugin strength is divided by the maximum actin level, which results in a infinite dH.
• Setting the strength of the plugin to 0 and resetting the actin levels to 0 just before the protrusion plugin becomes active works. However, this slows down the simulation and is quite complicated. Furthermore, the recorded actin levels are incorrect which makes it much harder to visualize the cell with actin leves.

Therefore, I changed adapted protrusion.cpp such that a maximum actin level of 0 results in dH = 0, see attached patch for more details.