Fix division by zero error when a dim is size 1

[kamesy]

When one of the sizes in the header is 1, startstoplen divides by 0. An example header:

...
dimension: 4
space: right-anterior-superior
sizes: 64 128 128 1
space directions: none (-0.12,0,0) (0,0,0.12) (0,0.12,0)
kinds: list domain domain domain
...

I ended up replacing startstep/startstop altogether in favour of Base.range.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (9578df7) 86.64% compared to head (d0e837f) 85.85%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   86.64%   85.85%   -0.80%     
==========================================
  Files           1        1              
  Lines         644      608      -36     
==========================================
- Hits          558      522      -36     
  Misses         86       86              

Files	Coverage Δ
src/NRRD.jl	85.85% <94.33%> (-0.80%)	⬇️

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[sjkelly]

Hi, thank you for the contribution! Can you please separate out the style/formatting changes from functional changes?