Improve creating scalar and null dataspaces.

[1uc]

Currently one needs to

auto dataspace = DataSpace(DataSpace::dataspace_scalar);

to create a scalar dataspace. Note that,

auto dataspace = DataSpace{DataSpace::dataspace_scalar};

creates a one-dimensional simple dataspace with of shape [0]. It would be nicer to allow something like:

auto dataspace = DataSpace::Scalar();
auto dataspace = ScalarSpace();
auto dataspace = DataSpace::createScalar();

[1uc]

Removing/changing DataSpace::dataspace_scalar might be something to consider for v3.

[1uc]

Complete since merging: #900.