Add caravel floorplan with a preliminary seal ring

- Use scripts/addmpwseal.tcl to reproduce
diff --git a/scripts/addmpwseal.tcl b/scripts/addmpwseal.tcl
new file mode 100644
index 0000000..7874eff
--- /dev/null
+++ b/scripts/addmpwseal.tcl
@@ -0,0 +1,10 @@
+drc off
+gds readonly true
+gds rescale false
+gds read ../gds/sram_1rw1r_32_256_8_sky130_lp1.gds
+load ./caravel.mag
+select top cell
+move origin -7.165um -7.120um
+box position 0 0
+getcell advSeal_6um_gen
+gds write caravel.mpw.gds