allow custom HDF5 data types in h5createDataset and h5writeDataset

[ilia-kats]

This useful when one has a custom type, but still wants to use the default HDF5 spaces, DCPLs etc. that these functions provide. For example, when writing logical vectors that are compatible with Python's h5py, one needs to create a custom enum type with H5Tenum_create.

[ilia-kats]

AFAICT the CI is failing somewhere in the testthat package itself, not due to my changes.

[codecov[bot]]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (e2e4ef7) 79.03% compared to head (f32940d) 78.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #130      +/-   ##
==========================================
- Coverage   79.03%   78.70%   -0.34%     
==========================================
  Files          36       36              
  Lines        1994     2005      +11     
==========================================
+ Hits         1576     1578       +2     
- Misses        418      427       +9     

Files	Coverage Δ
R/h5create.R	83.14% <40.00%> (-1.34%)	:arrow_down:
R/h5write.R	81.04% <57.14%> (-3.21%)	:arrow_down:

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