Make plink and bgen readers more consistent

[eric-czech]

This would include the following:

• Use the same code for contig interpretation
• Make the docs consistent
• Allow the bgen file to be supplied with no extension?
• Remove the dataframe-specific dtypes from the plink reader field lists
• Update plink test function names
• Remove lock option

[tomwhite]

Some of these apply to VCF too.

[tomwhite]

This and #629 are the only issues remaining for the 0.3.0 release. @eric-czech do you think you'll have a chance to look at this, or should we go ahead with the release without it?

[eric-czech]

I don't think I'll be able to get to it soon, unfortunately. They aren't critical changes so +1 to doing the release without this.