Consider using an enum for secondary structure

[timholy]

As pointed out in #65, mmCIF now annotates secondary structure in ways that don't fit into the current Char scheme. One way to expand it in a way that could enable support for both styles of annotation would be to use an @enum for secondary structure, and then have convenient conversion functions from Char.

[jgreener64]

If this will be useful to people then I can see the appeal. I must admit I didn't know about the more detailed mmCIF annotation values until recently though, and I'm not sure how many people use them.