hdf5: start handleing references

In earlier discussions, such as https://github.com/gonum/hdf5/pull/15, we talked about about working with an "HDF5 DataType that describes a pointer to x". As it is right now, whenever we are receiving a pointer, we are dereferencing it and are working with the underlying value.

The code that was at the center of the discussion was:

when creating a `DataType`
```go
case reflect.Ptr:
return NewDataTypeFromType(t.Elem())
```

and when performing encoding in our `cmem` package:
```go
case reflect.Ptr:
return enc.Encode(rv.Elem())
```

Looking at the [HDF5 Documentation](https://support.hdfgroup.org/HDF5/doc/UG/HDF5_Users_Guide-Responsive%20HTML5/index.html#t=HDF5_Users_Guide%2FDatatypes%2FHDF5_Datatypes.htm%23TOC_6_6_2_Referencebc-17&rhtocid=6.4.0_2) describing that topic, one notices that this is a bit more complex than we thought in our discussions. The guys there have a separate interface for handling the issue: [Reference Interface](https://support.hdfgroup.org/HDF5/doc/RM/RM_H5R.html).

Therefore, if we want to support those referencing types, we would need to create a wrapper for the named interface.

What still would not be resolved by adding the interface, is the issue of what happens if someone would try to e.g. append a pointer to a Go value without going through such an interface. Would we error? Or would we keep the status quo?

No file or test is named. Start at NewDataTypeFromType and the reflect.Ptr handling in the cmem encoding package, then review the linked HDF5 Reference Interface documentation and discussion. Done requires an agreed API for references and a decided behavior for pointers passed without that interface.

Written by the indexing model from the issue text.