Change StateVector indexing

[erogers-dstl]

This pull request updates StateVector indexing and code that uses it to demonstrate the change suggested in issue #186. I think this is a simple and useful update, but if people feel this over-complicates the Statevector class for little benefit, then the PR can be shelved.

[codecov-io]

Codecov Report

Merging #187 into master will increase coverage by 0.00%.
The diff coverage is 95.23%.

@@           Coverage Diff           @@
##           master     #187   +/-   ##
=======================================
  Coverage   91.90%   91.90%           
=======================================
  Files         111      111           
  Lines        3793     3794    +1     
  Branches      548      549    +1     
=======================================
+ Hits         3486     3487    +1     
  Misses        225      225           
  Partials       82       82           

Impacted Files	Coverage Δ
stonesoup/feeder/geo.py	90.56% <ø> (ø)
stonesoup/models/measurement/nonlinear.py	98.50% <92.30%> (-0.05%)	⬇️
stonesoup/functions.py	91.93% <100.00%> (+0.06%)	⬆️
stonesoup/sensor/radar/radar.py	95.83% <100.00%> (ø)
stonesoup/types/array.py	95.23% <100.00%> (+0.50%)	⬆️

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[jmbarr]

How does this work with types which aren't int but could usefully be returned directly (e.g. float, long, angle, myOwnVerySpecialType, etc)?

[jmbarr]

Doesn't this PR have much the same intent as #170? Would it be worth combining?

[erogers-dstl]

How does this work with types which aren't int but could usefully be returned directly (e.g. float, long, angle, myOwnVerySpecialType, etc)?

This works the same regardless of the type dtype of the StateVector (which I think is what you are asking). The check for int in __getitem__ and __setitem__ is whether the index supplied is an int: i.e. we did my_state_vector[1]. The check is necessary so that all other index modes (my_state_vector[:], my_state_vector[1, 0], my_state_vector[[1,2]] etc) behave as expected.

[DaveKirkland]

I admit the code does look much cleaner with this change.