Add methods

[JustinShenk]

• Fixes errors in issue JOSS Review - Example in latest docs (Predicting Trajectories) #83 including documentation, a bug in the model training API, and added functionality (PCA)
• Removes unused requirements

[codecov]

Codecov Report

Merging #86 (f3e122c) into master (659c8e0) will decrease coverage by 0.68%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   77.25%   76.57%   -0.69%     
==========================================
  Files          26       26              
  Lines        2660     2685      +25     
==========================================
+ Hits         2055     2056       +1     
- Misses        605      629      +24     

Impacted Files	Coverage Δ
traja/dataset/dataset.py	100.00% <ø> (ø)
traja/plotting.py	54.71% <3.33%> (-2.08%)	⬇️
traja/models/utils.py	72.50% <50.00%> (ø)
traja/models/predictive_models/lstm.py	100.00% <100.00%> (ø)
traja/models/train.py	97.09% <100.00%> (ø)

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