ENH: first working version of slicerSMTK plugin (export only)

This MR is for merging with master the branch containing the first version of a working slicerSMTK plugin. Currently it provides functionality for exporting MRML scene to the smtk (json) file format used by aevaCMB and aeva-session. The exported file is saved with extension .aeva.smtk which can then be loaded into aevaCMB application.

Features included:

1. Export surface models as vtkPolyData (.vtp)
2. Export solid models as vtkUnstructuredGrid (.vtu)
3. Export Images/Volumes as vtkImageData (.vti)
4. Export scene as smtk json format (.aeva.smtk)
5. Individual component files (vtp, vtu, vti) are saved under a subfolder with the same name as the parent filename.
6. Components undergo transformation hardening before saving, thereby carrying forward any registrations/transformations applied within Slicer.
7. File names of component files are made more readable by using the user-specified MRML node name as a prefix to the file followed by the respective uuid. The prefix undergoes string sanitization to ensure that it is compatible as a file name.