meshing: support a step inside rectangular domain

[fnrizzi]

• the simple meshing file needs to be extented to support a step inside

• the mesh for the domain wth the step should still be uniform

[fnrizzi]

5                    4
o--------------------o
|                    |  
|          2         |    
|        o----------o 3
|        | 
|        |
o--------o
0         1

time python create_full_mesh.py --outDir $HOME/Documents/tmpFull -s 3 \
  --bounds x0 y0 x1 y1 x2 y2 x3 y3 x4 y4 x5 y5  \
  --periodic false -n 90 30 -p show 0