Speed up printing of large sample sets

[arcondello]

Comparison:

import time

import dimod
import numpy as np

sampleset = dimod.SampleSet.from_samples(np.ones((91, 500000)), vartype='BINARY', energy=np.zeros(91))

t = time.perf_counter()
dimod.serialization.format.Formatter().format(sampleset)
print(time.perf_counter() - t)

As an example, running this simple test locally gives 22.33913665200089 without the change and 0.007365263998508453 with.

Also add variable object slicing.

Closes #1069
Closes #1068

[codecov-commenter]

Codecov Report

Merging #1075 (9871201) into main (52901df) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1075   +/-   ##
=======================================
  Coverage   92.86%   92.87%           
=======================================
  Files          81       81           
  Lines        7349     7354    +5     
=======================================
+ Hits         6825     6830    +5     
  Misses        524      524           

Impacted Files	Coverage Δ
dimod/serialization/format.py	97.57% <100.00%> (+0.06%)	⬆️

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[seatim]

Excellent work!