Enable parameter adds/subtracts

[nikhilwoodruff]

Fixes #68

[codecov]

Codecov Report

Base: 84.24% // Head: 84.11% // Decreases project coverage by -0.13% ⚠️

Coverage data is based on head (1236b42) compared to base (0aeae3f).
Patch coverage: 5.88% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   84.24%   84.11%   -0.14%     
==========================================
  Files         184      184              
  Lines        8685     8699      +14     
  Branches      989      991       +2     
==========================================
  Hits         7317     7317              
- Misses       1120     1134      +14     
  Partials      248      248              

Impacted Files	Coverage Δ
policyengine_core/simulations/simulation.py	55.80% <0.00%> (-1.84%)	⬇️
policyengine_core/variables/variable.py	84.80% <100.00%> (ø)

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