Adding functionality to insert until confluence

[allison-li-1016]

Review time: s

Context: Currently insert only adds 1 cell per location. Certain cell types allow multiple cells to occupy the same location. I want to add the option to insert cells into the same location until the cell's density limit is reached.

Summary of changes:
Insert class

• Added parameter for confluence option
• Added loop to insert until max density is reached
• Refactored cell initializing and adding into helper methods

PatchCell class

• Added helper function to return max density per cell. We can discuss this implementation since technically max density should be per population not cell.
• Made checkLocation public so other classes can cell the method. We would not need to do this if this method was refactored into the utils class instead, which I think is a WIP PR or Issue already (Factored checkLocationSpace into multiple methods in PatchUtilities #224)

Example usage:

<action id="INSERT_HIGH_DENSITY_TUMOR" class="insert">
     <action.parameter id="TIME_DELAY" value="0" />
     <action.parameter id="INSERT_NUMBER" value="3000" />
     <action.parameter id="INSERT_RADIUS" value="14" />
      <action.parameter id="CONFLUENCE" value="1" />
     <action.register id="cancer" />
 </action>

[Jannetty]

LGTM! Made some suggestions mostly for logging when the code defaults to something other than what the user specified. At some point I think it could be nice to have the ability to insert intermediate numbers of cells to each location (values other than 1 and the max), but I don't think that has to be in this PR. Great work!

[kristaphommatha]

LGTM! Just a small thing about the way the confluence parameter is read

[kristaphommatha]

LGTM!