Remove Property Loader Source Location File Requirement

[--thatguy]

The property loader step allows the source location to be defined via an expression.  However, if that expression does not evaluate to a file on disk at compile time you get an error.  This isn't always desired behavior, for instance, when used in a plugin architecture.

The current workaround is to include a dummy file which could unnecessarily complicate the software & deployments.  A dummy file also has the potential to mask errors that should be presented to the user. 

The only validation TestStand does of the property loader source location file is that it exists.  It doesn't do any validation on the file contents.  So is there any benefit?  TestStand properly throws an error if the expression doesn't evaluate to a valid file.

Alternatively, a developer could deselect the sequence analyzer rule "Property Loader source should be proper", but this would disable it for all analysis not just the ones that use expressions