Empty private field header for dicomtocsv

[dgobbi]

When the dicomtocsv "-k" option is given a private tag, the name that it generates for the csv header is as follows:

1. If the private tag is found in a private dictionary, and has a name, then that name is used.
2. If the private tag is found in a private dictionary, but has no name, then "Unknown" is used.
3. If the private tag is not found in a dictionary, then the name is blank (empty).

I consider (3) to be a bug. The header name should be the same for (2) and (3).

The use of "Unknown" is not ideal, either, because can result in multiple fields with the same name "Unknown". These fields can only be distinguished by the order in which they occur. Other possibilities are:

1. Name these fields "Private1", "Private2", etc. using the column index.
2. Use the full private tag specification (the Creator name and the hexadecimal tag). This will be a long name!
3. Use just the hexadecimal tag. Without the creator name, this is an incomplete specification, but better than nothing.
4. Provide some way for the user to specify the name to use, e.g. extend the "-k" syntax to allow this.

For (4), there may already be comments in a previous issue, so search those first!