Save subregions to json and reorganise IO.

[lang-m]

• Save subregions to json file when saving a field
• Load subregion information from json file (if it exists) when loading a field from disk
• Move IO for df.Field to a separate submodule
• Use pathlib.Path for io

[codecov-commenter]

Codecov Report

Merging #158 (63f7859) into master (7b8019d) will decrease coverage by 0.10%.
The diff coverage is 97.87%.

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
- Coverage   95.86%   95.76%   -0.11%     
==========================================
  Files          22       27       +5     
  Lines        2371     2433      +62     
==========================================
+ Hits         2273     2330      +57     
- Misses         98      103       +5     

Impacted Files	Coverage Δ
discretisedfield/io/ovf2vtk.py	100.00% <ø> (ø)
discretisedfield/util/__init__.py	100.00% <ø> (ø)
discretisedfield/util/util.py	81.57% <ø> (-6.04%)	⬇️
discretisedfield/io/util.py	58.33% <58.33%> (ø)
discretisedfield/io/ovf.py	99.08% <99.08%> (ø)
discretisedfield/field.py	96.98% <100.00%> (-0.50%)	⬇️
discretisedfield/io/__init__.py	100.00% <100.00%> (ø)
discretisedfield/io/hdf5.py	100.00% <100.00%> (ø)
discretisedfield/io/vtk.py	100.00% <100.00%> (ø)
discretisedfield/mesh.py	99.36% <100.00%> (+0.01%)	⬆️
... and 1 more

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[swapneelap]

Added a couple of comments. Everything else looks good to me.