Storing variable-length strings in HDF5 format

[WardLT]

Our current HDF5 writer saves strings columns as fixed length, using the maximum length of any string to set the string lengths. This only works when you have seen all possible values of the the string, which is not true if you're appending to an existing HDF5 file.

We should explore ways of at least warning users of this behavior and, ideally, providing ways around it. Ideas include

• Warning users if new strings exceed the fixed length
• Letting users set an expected maximum for lengths
• Storing string columns in separate, VLArray storage
• Rebuilding a Table with a new, longer column if a new entry is found

[victorventuri]

The options "Let user set the expected max" and "Rebuild Table" can likely be done with tables.StringCol

The option VLArray kind of necessitates the same thing, as even there, we need to specify the maximum number of characters allowed in the string.

I can't speak to the cost of rebuilding the table as needed, but I imagine it would be nice if that is an option the user can set. Maybe we set it to False as default, but, if the user desires, they can turn it on?