Implement a basic HDF5 table writing class.

Implement a class, HDFTable, that can do buffered per-row writes into
a HDF5 table. Demo in tests/hdftable.cc
Add supporting functions for strings.
Always separate buffer and current data.

It would make sense not to have a separate buffer if buffer_size==1,
but that means memcpy() in write() would need a check (although it
seems to work without it as well). In the interests of more simpler
code, it is better to always copy stuff into a buffer. The idea is
anyway that the class should act as a reasonable IO buffer.
Set the chunk size to 1000 for HDFTable.
Make HDFTable::bind() const.

Internally use map::find(), because that supports const.
Use HDFTable for fgamma.

Remove HDFWriter in process -- no need for that anymore.
Remove an unnecessary tab.