New format tutorial

[cczhu]

(A fairly significant) leftover from #142 is to create a tutorial on how to support new formats.

Note that if we abstract DADA and GUPPI (#234), it would be wise to write the tutorial afterwards, since it should describe Baseband's file and class inheritance structure.

[mhvk]

I hope this can be for the ASP format that @misanthropealoupe is implementing: https://github.com/misanthropealoupe/baseband/tree/asp