Optional BiblicalTermsListSetting element in the Settings.xml file

[mshannon-sil]

From issue 543 in SILNLP: "machine/corpora/paratext_project_settings_parser_base.py treats the BiblicalTermsListSetting element in the Settings.xml file as a mandatory element. While Paratext projects will have this element, it is sometimes necessary to work with projects that do not have any Settings.xml file, and a simple Settings.xml file must be hand-constructed for it. Treating the BiblicalTermsListSetting element as mandatory means that this is another XML element that has to be provided in the hand-constructed file. Treating this as optional file (perhaps with a warning message) would be convenient."

Since machine.py will be changing, we'll need to make a corresponding change in Machine in src/SIL.Machine/Corpora/ParatextProjectSettingsParserBase.cs.