Remove `varargout` in remaining functions

[dominikbach]

Summary

All functions should return static output (i.e. no use of varargout). Some functions do not do this, including pspm_overwrite and pspm_prepdata. This requires checking the entire codebase where these functions are called.

pspm_overwrite

1. This is an internal function with no user-specified arguments. Thus, the output argument sts is not needed, and we can settle on one static output argument.
2. There should be a third value for overwrite which signifies that the argument has not been set by the user.
3. In case of this third value, the GUI should be enabled in case the file exists.
4. pspm_options should use this third value as default.
5. pspm_options should check the field .overwrite for all (and only for these) functions that create new files.
6. Check all instances in PsPM where pspm_overwrite is called and ensure the output arguments are set appropriately.

pspm_prepdata

1. This is an internal function but it checks a user-supplied struct variable and so requires input checking and a sts output.
2. Thus, ensure a static output [sts, data, newsr].
3. Check all instances in PsPM where pspm_prepdata is called and ensure the output arguments are set appropriately.

Other functions

Check all PsPM functions in the src folder and list all that use varargout.