Ensure only correct galley genres are considered in full-text parsing

[ewhanson]

Problem

Currently galley genre is not considered when selecting with galley to parse for creating the full-text to be included in the JATS XML. This means its possible other supplementary files could be parsed/given priority over the full-text files in rare cases.

Solution

Filter galley files by genre to ensure only the correct types of genres are being considered.

Notes

This is not currently a problem that exists in recorded practice, but if it does become a problem with the completion of #59, this issue should be considered.