Changed FunctionBase FilesystemContext base to working directory
* this changes filesystem context resolution for external function execution to be relative to the working directory
** this was changed to enable module support in BuildXSLT
*** ... which is in turn required as a foundation for extracting the detail transformations into a separate "StaticXSLT" application
* added parameter-less FilesystemContext constructor which defaults to the working directory
Split path attribute of FunctionReadFile result tree into base and name
* this is required to be able to resolve paths contained within a file relative to its location
** which is in turn needed to enable BuildXSLT modules without requiring the module transformations to be resolved using IncludeEntityResolver