Print parents when printing GaussianConditional

[varunagrawal]

Currently, GaussianConditionals don't print the parents, only the frontals. This PR fixes that.

For the following conditional

auto conditional =
      GaussianConditional::FromMeanAndStddev(X(0), A1, X(1), b, sigma);

which is just P(X0 | X1).

Before

GaussianConditional  Conditional density [x0] 
  R = [ 1 0 ]
      [ 0 1 ]
  S[x1] = [ -1 -2 ]
          [ -3 -4 ]
  d = [ 20 40 ]
isotropic dim=2 sigma=3

After

Conditional density [x0 | x1]
  R = [ 1 0 ]
      [ 0 1 ]
  S[x1] = [ -1 -2 ]
          [ -3 -4 ]
  d = [ 20 40 ]
isotropic dim=2 sigma=3

[varunagrawal]

NOTE: I've converted the default print pre-text to empty string rather than GaussianConditional (since it is already printing Conditional density). Please let me know if I should undo this.

[dellaert]

NOTE: I've converted the default print pre-text to empty string rather than GaussianConditional (since it is already printing Conditional density). Please let me know if I should undo this.

I would prefer if you keep the default, but remove the redundant Conditional density. Also, maybe actually do print p(x0 | x1). Note lowercase p because it's a density, not a probability mass function.

[varunagrawal]

@dellaert done, here is the new output:

GaussianConditional p(x0 | y0 y1)
  R = [ 1 0 ]
      [ 0 1 ]
  S[y0] = [ -1 -2 ]
          [ -3 -4 ]
  S[y1] = [ -5 -6 ]
          [ -7 -8 ]
  d = [ 20 40 ]
isotropic dim=2 sigma=3

[dellaert]

Cool beans!