Add "OpenEXR" package about hunter

[rbsheth]

@hjmallon It would be nice to split IlmBase out but that will be a nightmare to maintain, so I'm okay with your proposed steps.

[rbsheth]

@hjmallon Thanks for getting some PRs upstream! I made a fork here: https://github.com/cpp-pm/openexr

[hjmallon]

cpp-pm/openexr#1

Added hunterization PR to that fork.

[hjmallon]

Note to self, tasks required:

• Merge hunterisation here 2.4.0-p0 cpp-pm/openexr#1
• Make PR based on that. pr.IlmBase for IlmBase
• Wait for release with IlmBase
• Add testing PR for IlmBase
• Make v2.4.0-p1 with a Hunter release including IlmBase
• Make PR based on that pr.OpenEXR which also includes pointing IlmBase at version 2.4.0-p1
• Wait for release with OpenEXR and IlmBase
• Add testing PR for OpenEXR

Adding packages with 2 dependant packages inside them is a little complicated as the hunterisation of a package includes a link to a version of hunter that itself supports building part of the package...

[hjmallon]

@hjmallon It would be nice to split IlmBase out but that will be a nightmare to maintain, so I'm okay with your proposed steps.

Quote from @cary-ilm

Our intention with the 3.0 release later in the year is to split these projects into separate github repos, creating proper dependencies between them, which will mean an entire re-vamp of the cmake setup.

As mentioned here AcademySoftwareFoundation/openexr#639 the OpenEXR are planning to split IlmBase and OpenEXR in a v3.0.0 release this year anyway. So the weird dependency will be sorted then.

[rbsheth]

@hjmallon Closing as #138 and #164 have been merged