Add latlongimage node

[pablode]

This PR proposes an initial version of a node that samples equiangular images.

Question: should the image framerange/frameoffset/frameendaction inputs be forwarded?

[jstone-lucasfilm]

This looks like a great step forward to me, @pablode, and I believe the testing errors just require the same fix as you implemented for fract, where each type variant of the new node has a single usage in our Materials/TestSuite folder.

[pablode]

Not sure about that, it seems rather an issue with the Unit system not being set up correctly in the test runner. (See log messages and 2nd commit succeeding.)

[jstone-lucasfilm]

@pablode Ah, good catch, and I was making assumptions about these errors being the same as the ones in fract.

As a starting point, perhaps we should omit the new unit attribute in the latlongimage node, and we can log that as a future improvement that would be linked to updates to our test suite?

[pablode]

Sure, reverted the commit. Maybe in the future, this can be an 'idiomatic' MaterialX practice ;)