Some basic house keeping

[Tokazama]

Changes:

• NIVolume and NIfTI1Extension don't need to be mutable
• use parsing functions instead of slower dictionary look up for converting header info
• use updated workflows for coverage tests

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@e72842f). Click here to learn what that means.
The diff coverage is 23.07%.

@@            Coverage Diff            @@
##             master      #49   +/-   ##
=========================================
  Coverage          ?   43.69%           
=========================================
  Files             ?        2           
  Lines             ?      357           
  Branches          ?        0           
=========================================
  Hits              ?      156           
  Misses            ?      201           
  Partials          ?        0           

Impacted Files	Coverage Δ
src/parsers.jl	8.16% <8.16%> (ø)
src/NIfTI.jl	68.57% <85.71%> (ø)

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