examples/relativity.py -- convert output to pprint (#719)

examples/relativity.py -- convert output to pprint  (#719)

This is how it now looks like:

----------------------------------------
Christoffel symbols:
       d
       ──(ν(r))
       dr
Γ⁰₁₀ = ────────
          2
       d
       ──(ν(r))
       dr
Γ⁰₀₁ = ────────
          2

        -λ(r) + ν(r) d
       ℯ            ⋅──(ν(r))
                     dr
Γ¹₀₀ = ──────────────────────

...

----------------------------------------
Ricci tensor:

...

----------------------------------------
solve the Einstein's equations:
           ⎛     C₂⎞
λ(r) = -log⎜C₁ + ──⎟
           ⎝     r ⎠
metric:
⎡      C₂                         ⎤
⎢-C₁ - ──     0     0       0     ⎥
⎢      r                          ⎥
⎢                                 ⎥
⎢             1                   ⎥
⎢   0      ───────  0       0     ⎥
⎢               C₂                ⎥
⎢          C₁ + ──                ⎥
⎢               r                 ⎥
⎢                                 ⎥
⎢                    2            ⎥
⎢   0         0     r       0     ⎥
⎢                                 ⎥
⎢                        2    2   ⎥
⎣   0         0     0   r ⋅sin (θ)⎦

Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>
Signed-off-by: Ondrej Certik <ondrej@certik.cz>