Updating VTK9 fix to remove specific VTK9 option

This MR builds on the changes by @kayarre (thanks for the initial fix @kayarre) to remove the specific NEKTAR_USE_VTK9 option and instead to find the most recently available version of VTK on the target system.

See the discussion in MR1300 in the main Nektar++ repository for further information.

A couple of additional points on this update:

• From testing builds with different versions of VTK, given the updates in the original fix submission, the NEKTAR_USE_VTK option successfully finds VTK9 even without specifying NEKTAR_USE_VTK9 (although the original module names have been updated, the old names are still available but generate deprecation warnings).
• Some users working with systems with packaged versions of VTK may not actually know what version of VTK is installed on their system without further investigation.

As a result, it was felt that a simpler, non-version-specific approach might be easier in this case.

This MR also adds a couple of fixes to add required headers to the VtkToPng and VtkToFld tools so that they build with VTK9.

Hope this all sounds OK with you @kayarre. Happy to discuss, otherwise if this is OK, please merge these changes into yours and I guess we should then be able to merge everything into the main repo under the original MR - wanted to maintain the history with your contribution.