Small enhancements for Spice reader

- Detects recursive subcircuit calls now
- Dismisses empty top level circuit which happened to be created
  when there were not top level elements and control statements
  were present (such as .param)

testdata/algo/nreader24.cir

@@ -0,0 +1,14 @@
+* Testing recursive call detection
+
+.subckt c1 a b
+x1 a b c2
+.end
+
+.subckt c2 a b
+x1 a b c1
+.ends
+
+xtop vdd vss c2
+
+.end
+

testdata/algo/nreader25.cir

@@ -0,0 +1,11 @@
+* Test: dismiss empty top level circuit
+
+.subckt top a b 
+m1 a b a b nmos
+.ends
+
+* this triggered generation of a top level circuit
+.param p1 17
+
+.end
+