Cast attributes to string if user specifies string

[bryan-harter]

I believe right now in _convert_data, if you see something like this:

_convert_data: data_type = 51 , num_elems = 1 , num_values = 1 , indata = 255

then we have an issue where the number "255" is not converted to string, and writing the CDF fails. If a user specifies string, we probably should cast it to a string.