Added an example for adding matrices.

examples/add.pir

@@ -0,0 +1,29 @@
+.sub main :main
+  .local pmc lib
+
+  lib = loadlib "NumMatrix2D"
+  unless lib goto not_loaded
+  say "library loaded successfully."
+
+  $P1 = new 'NumMatrix2D'
+  $P1.'resize'(5,5)
+  $P1[1;1] = 4.
+  $S1 = $P1
+  say $S1
+
+  $P2 = new 'NumMatrix2D'
+  $P2.'resize'(5,5)
+  $P2[1;1] = 3.
+  $P2[1;2] = 4.
+  $S2 = $P2
+  say $S2
+
+  $P3 = $P1 + $P2
+  $S3 = $P3
+  say $S3
+
+  end
+
+not_loaded:
+  say "Could not load library."
+.end