Add variable_dimensions methods

[lrennels]

@jrising

[codecov-io]

Codecov Report

Merging #589 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
+ Coverage      79%   79.16%   +0.16%     
==========================================
  Files          39       39              
  Lines        2720     2722       +2     
==========================================
+ Hits         2149     2155       +6     
+ Misses        571      567       -4

Impacted Files	Coverage Δ
src/core/types/time.jl	100% <ø> (ø)	⬆️
src/core/model.jl	81.81% <ø> (ø)	⬆️
src/core/defs.jl	80.06% <100%> (+1.4%)	⬆️

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[lrennels]

@ckingdon95 I've addressed the requested changes thank you!

[corakingdon]

Sorry didn't catch this till I saw you correct it on the other PR, but this one should be TimestepIndex

[lrennels]

@ckingdon95 thanks just fixed this, so you're right this encompasses both PRs ... I'll close the docstrings one and wait for tests to pass on this one. If it looks good now you can approve and we'll be good to go sorry for confusion!