documentation for the data module

[aryarm]

we already have the automatically generated documentation in the API section

but it would be nice if we had a basic tutorial page sorta like pysam's

specifically, we should discuss

• how to instantiate an instance
• what kinds of properties each class has
• how to read a file
  • and how to iterate over a file
• a brief overview of some of the methods in each class
• how to write a file
• how to extend each class to create your own custom classes

For the Haplotypes class, we should be able to more-or-less copy the Details section of #43

[aryarm]

Resolved by #67