Can't parse a usfm file that uses .GRK as the file extension.

[johnml1135]

Spin off from sillsdev/silnlp#604 to implement for Machine.py.

Original finding from David Baines:

The file extension is given in the project's Settings.XML file and typically includes ".SFM" in the 'PostPart' of the file name.

For the Greek resource the USFM files have '.GRK' as their file extension and the line in the Settings.XML is:
Naming PostPart=".GRK" BookNameForm="41MAT">
Using silnlp.common.check_books to check the GRK project fails to find any SFM files.

The Danish Bible DBSV uses '.PT7' as the file extension for the USFM files. These can't be found by the check_books script though the line in the settings file seems to be correct:

[ddaspit]

This is not an issue with Machine.