Add ability to add comments to png images.

[brugger1]

I added the ability to add comments to png images. The comments are visible in XV.

I added the ability to store comments to the built in PNGEncoder. I added 2 new Encode methods with an extra argument containing the comments. The comments consist of a key and a string. I went with a vector of strings. The number of strings needs to be a multiple of 2. Right now the existing Encode methods still need to exist. Ascent could be modified to use only the new ones.

I removed an unnecessary #include PNGEncoder.h from Renderer and ScalarRenderer since they weren't necessary.

I added Set/GetComments to Render as well as logic to pass them to PNGEncoder.

For compositor/Image, I added another Save method that took the comments in addition to the file name. This seemed like the more natural way to do it for that class.

I've tested this with Ascent running the lulesh proxie in serial. This hasn't been tested with other proxies or using Rover or in parallel.

Here is an image from the resulting file showing the comment in XV.

Since this is my first commit to vtk-h/ascent, I'm expecting a fair number of requests for changes.

[mclarsen]

How hard is it to add the ability to read the comments from a png?

[mclarsen]

I guess we don't actually have the decoder inside vtkh, but it would be useful to add in ascent.