UTF-8 encoding of all strings for names and contents

[gicmo]

This enables utf8 encoding for the names of groups, datasets and links as well as for all string values in datasets and attributes. Closes #779

[codecov[bot]]

Codecov Report

Merging #810 into master will increase coverage by 0.16%. The diff coverage is 97.75%.

@@            Coverage Diff             @@
##           master     #810      +/-   ##
==========================================
+ Coverage   94.74%   94.90%   +0.16%     
==========================================
  Files         179      181       +2     
  Lines       12545    12609      +64     
==========================================
+ Hits        11886    11967      +81     
+ Misses        659      642      -17     

Impacted Files	Coverage Δ
backend/hdf5/h5x/Attribute.hpp	100.00% <ø> (ø)
backend/hdf5/h5x/H5DataType.hpp	100.00% <ø> (ø)
backend/hdf5/h5x/H5Group.hpp	100.00% <ø> (ø)
backend/hdf5/h5x/LocID.hpp	100.00% <ø> (ø)
backend/hdf5/h5x/H5Group.cpp	92.04% <83.33%> (+8.71%)	:arrow_up:
backend/hdf5/h5x/H5Plist.cpp	94.44% <94.44%> (ø)
backend/hdf5/h5x/Attribute.cpp	96.96% <100.00%> (+0.96%)	:arrow_up:
backend/hdf5/h5x/H5DataType.cpp	80.46% <100.00%> (+0.65%)	:arrow_up:
backend/hdf5/h5x/H5PList.hpp	100.00% <100.00%> (ø)
backend/hdf5/h5x/LocID.cpp	100.00% <100.00%> (ø)
... and 4 more

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[gicmo]

Almost good to go, I want another tests that checks all the link names, attributes and contents have the correct character set.

[jgrewe]

lgtm, @gicmo will you rebase it or should I doe the update here?