Add named ctors for scalar and null dataspaces.

[1uc]

The issue is that:

DataSpace{DataSpace::dataspace_scalar};
DataSpace{DataSpace::dataspace_null};

are a simple dataspace with 0 elements and a simple dataspace with one element, respectively. This provides a safer alternative.

[1uc]

See:

• Improve creating scalar and null dataspaces. #747
• Writing fixed-length string with Ascii encoding #678 (comment)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c660174) 85.34% compared to head (6acdc79) 85.38%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
include/highfive/bits/h5s_wrapper.hpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #899      +/-   ##
==========================================
+ Coverage   85.34%   85.38%   +0.03%     
==========================================
  Files          84       84              
  Lines        5508     5527      +19     
==========================================
+ Hits         4701     4719      +18     
- Misses        807      808       +1     

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[1uc]

Precedence:

HighFive/include/highfive/H5PropertyList.hpp

Line 178 in 9e5cb5b

static const PropertyList<T>& Default() noexcept {

HighFive/include/highfive/H5DataSpace.hpp

Line 218 in 9e5cb5b

static DataSpace From(const T& value);

HighFive/include/highfive/H5DataSpace.hpp

Line 230 in 9e5cb5b

static DataSpace FromCharArrayStrings(const char (&string_array)[N][Width]);