fix: Use python for arithmetic in Makefile

Shell doesn't understand scientific notation and POSIX arithmetic is
extremely brittle.

examples/fluidity/corner2d/Makefile

@@ -10,6 +10,8 @@ OUT_DIR := _out
 # Initial geometry options (in units of metres), passed to `pydrex-mesh`.
 WIDTH := 1e6
 DEPTH := 2e5
+HALF_WIDTH := $(shell python3 -c 'print($(WIDTH)/2)')
+HALF_DEPTH := $(shell python3 -c 'print($(DEPTH)/2)')
 RESOLUTION_HI := 1e2
 RESOLUTION_LO := 1e4
 # Initial conditions, used to parametrise velocity fields and spawn particles.
@@ -57,7 +59,7 @@ $(OUT_DIR)/$(SIM_NAME).msh:
 	@echo "********** Building the mesh file..."
 	./envcheck.sh -m
 	pydrex-mesh -k="rectangle" -a xy $(WIDTH),$(DEPTH) \
-		-c $$(($(WIDTH)/2)),$$((-$(DEPTH)/2)) -r NW:$(RESOLUTION_HI),SE:$(RESOLUTION_LO) $@
+		-c $(HALF_WIDTH),$(HALF_DEPTH) -r NW:$(RESOLUTION_HI),SE:$(RESOLUTION_LO) $@
 
 .PHONY: clean
 clean: