Create new variable-length data type
vlen_type_id = H5T.vlen_create(base_id)
vlen_type_id = H5T.vlen_create(base_id) creates
a new variable-length (VL) data type. base_id specifies
the base type of the data type to create.
Create a variable length data type for 64-bit floating-point numbers.
base_type_id = H5T.copy('H5T_NATIVE_DOUBLE');
vlen_type_id = H5T.vlen_create(base_type_id);