render values in markdown

[dellaert]

Added an optional argument "names" to discrete markdown renderers.

An example from testDiscreteConditional.cpp:

  DecisionTreeFactor::Names names{
      {0, {"Zero", "One", "Two"}}, {1, {"-", "+"}}, {2, {"T", "F"}}};
  cout << conditional.markdown(formatter, names);

P(A|B,C):

B	C	T	F
-	Zero	0	1
-	One	0.25	0.75
-	Two	0.5	0.5
+	Zero	0.75	0.25
+	One	0	1
+	Two	1	0

update: I snuck in a fix for sampling in discrete as well, hope that's ok.

[varunagrawal]

Stray cout print in a test

[varunagrawal]

LGTM modulo 2 comments.