Make future loading optional in prediction

[teubert]

These changes make future loading optional for predictors that do not have an input (e.g., ThrownObject). This brings the behavior of predictors inline with simulation

[github-actions[bot]]

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• [ ] Reviewer (someone other than author) should look for bugs, efficiency, readability, testing, and coverage in examples (if relevant).
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[codecov-commenter]

Codecov Report

Merging #78 (a34172e) into dev (670e1cf) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev      #78   +/-   ##
=======================================
  Coverage   84.24%   84.24%           
=======================================
  Files          99       99           
  Lines       10157    10157           
=======================================
  Hits         8557     8557           
  Misses       1600     1600           

Files Changed	Coverage Δ
src/progpy/predictors/monte_carlo.py	91.66% <100.00%> (+0.17%)	:arrow_up:
src/progpy/predictors/unscented_transform.py	92.64% <100.00%> (+0.10%)	:arrow_up:
tests/test_predictors.py	96.69% <100.00%> (-0.05%)	:arrow_down: