Generates a template constrained to a circle.
TemplateRandomCircularGrid[nmin, radius] returns a tissue with at least nmin cells and a total radius as specified.
The total number of cells will be the smallest exact square integer larger than or equal to nmin.
The cells are generate on a circular polar grid with concentric circles of cell centers. The first cell is placed at the origin; this is surrounded by a circle of 3 evenly spaced cells; which is subsequently surrounded by circles of 5, 7, 9, ... evenly spaced cells until nmin is reached. Then all the cell centers are randomply perturbed and cell walls are determined as a constrained Voronoi.