Flexible std::strings.

[1uc]

This PR outlines how one could read and write std::string and containers of std::string into/from fixed or variable length HDF5 strings.

[codecov]

Codecov Report

Attention: 35 lines in your changes are missing coverage. Please review.

Comparison is base (386d1b6) 84.68% compared to head (8bfa11f) 84.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #744      +/-   ##
==========================================
+ Coverage   84.68%   84.73%   +0.04%     
==========================================
  Files          68       69       +1     
  Lines        4812     5043     +231     
==========================================
+ Hits         4075     4273     +198     
- Misses        737      770      +33     

Files	Coverage Δ
include/highfive/bits/H5Attribute_misc.hpp	80.30% <100.00%> (ø)
include/highfive/bits/H5DataType_misc.hpp	79.59% <ø> (ø)
include/highfive/bits/H5Inspector_misc.hpp	90.97% <100.00%> (+0.30%)	⬆️
include/highfive/bits/H5ReadWrite_misc.hpp	83.87% <100.00%> (+3.22%)	⬆️
include/highfive/bits/H5Slice_traits_misc.hpp	90.24% <100.00%> (ø)
tests/unit/tests_high_five_base.cpp	99.71% <100.00%> (+0.01%)	⬆️
include/highfive/bits/H5Converter_misc.hpp	96.62% <95.16%> (-3.38%)	⬇️
src/examples/read_write_std_strings.cpp	0.00% <0.00%> (ø)

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[1uc]

Things to discuss:

• Currently the new code defaults to ASCII while old code default to UFT8. Probably, it would make more sense to default to UFT8 everywhere.
• There's no default for the padding of fixed-length strings. The issue is that the natural choice null-terminated could easily lead to writing strings to disk that claim they're null-terminated, but they're not. The fact that they're not null-terminated isn't the issue, we know their length. The problem is only that they claim that they are; when they're not.

[matz-e]

Had a glance over this, looks good.