Added tests and documentation for soa<> rate qualifier.

tests_errors/soa-7.ispc

@@ -0,0 +1,7 @@
+// Can't convert between types "uniform struct Point" and "soa<4> struct Point" with different SOA widths
+
+struct Point { float a, b, c; };
+
+void foo(soa<8> Point pts[]) {
+    soa<4> Point x = pts[0];
+}