Add option to specify output file directory for Fscan programs

Description

This MR adds the option --outputDir (default: ".") to Fscan programs so that an explicit path can be given separately from the --outputBname option. This was already the normal behaviour unless the outputBname started with something that looked like a path, e.g., --outputBname=/home/albert.einstein/my_output_basename.

With this MR, the default is for --outputDir=.. Thus if a code or script executes any of the Fscan programs and they specify a path in the outputBname option, they will need to change the code/script to move the path part into the outputDir option.

API Changes and Justification

I'm not sure if this change is backwards compatible or incompatible, as it doesn't modify class/function/struct/type definitions or adds anything new in C headers or python class/functions. However, it does cause other scripts calling these programs to pay attention whether they use a path in the outputBname option, if they use it.

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

@karl-wette @david-keitel should have a look. @keith-riles Should also check the existing online spectral monitoring code to see if outputBname is used, and if there is a path-like string at the beginning. We should coordinate to make sure this change is satisfactory.