Added ability to read function files to Plot3D reader.

The Plot3D reader now supports the ability to read function files.

Change-Id: Ic351dc2d6885e7015781006967956d38376b36cf
Several enhancements to the Plot3D readers.

- Added ability to read function files
- Minor cleanup
- Refactored internal class so that it can be used by
subclasses in the future.

Change-Id: I2eaa819ccace77d30379bc90f5f9956bcd1d58a7
Merge topic 'plot3d-function-support' into master

5a824a95 Several enhancements to the Plot3D readers.
3d8bbc74 Added ability to read function files to Plot3D reader.