nf_file_names should not have default value

[rabutler-usbr]

Previously I've been updating the default version = parameter. But doing this means that running nf_file_name() with two different versions of CRSSIO could result in two different sets of file names being created. Seems better to remove the default, and require user to provide it.

This should be a simple change, but the Shiny add in always uses the default version. So, will need to update the Shiny app.

Consider adding 'latest' as a valid argument. Then app and other functions could always use latest, but user would know that's the case.

• remove default in nf_file_names() and natsalt_file_names()
• update Shiny app
• tests

[rabutler-usbr]

added message to nf_file_names() that future versions will require specified version, if default value is used.

Using warning was a lot of trouble for all the tests that don't use a default value.