No support for fixed maxsize (at least using the C API)

[HealthyPear]

While exploring ways to implement what is being discussed in #830, we wanted to start by testing the writing of variable-length datasets (in this case using the C API - by the way is the C++ API supported for the test files?).

I took the example from the HDF5 docs
https://github.com/HDFGroup/hdf5-examples/blob/master/C/H5T/h5ex_t_vlen.c

Using it as is I get this with h5dump

HDF5 "h5ex_t_vlen.h5" {
GROUP "/" {
   DATASET "DS1" {
      DATATYPE  H5T_VLEN { H5T_STD_I32LE }
      DATASPACE  SIMPLE { ( 2 ) / ( 2 ) }
      DATA {
      (0): (3, 2, 1), (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144)
      }
   }
}
}

but than opening it with pytables or vitables results in this error

/Users/michele/Applications/PyTables/tables/group.py:310: UserWarning: leaf ``/DS1`` is of an unsupported type; it will become an ``UnImplemented`` node
  warnings.warn(
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'UnImplemented' object is not subscriptable

Comparing this to what can be done using pytables, e.g.
https://github.com/PyTables/PyTables/blob/master/examples/vlarray1.py

we discovered that if we set a maxsize as pytables produces (instead of NULL like in the original example)

hsize_t maxdims[1] = {H5S_UNLIMITED};

and then chunking the unlimited maxsize dataset

hid_t dcpl_id = H5Pcreate(H5P_DATASET_CREATE);
status = H5Pset_chunk(dcpl_id, 1, dims);
dset   = H5Dcreate(file, DATASET, filetype, space, H5P_DEFAULT, dcpl_id, H5P_DEFAULT);

we can read back the file with pytables / vitables

[avalentino]

Hi @HealthyPear
sorry, but I'm not sure to follow here.
Are you suggesting that we should improve the handling of maxsize = NULL?
In that case a simple code snippet + dataset demonstrating the issue would help a lot.
Even better if you submit a PR.