Remove exposure calculator code and move it into a plugin

What is the purpose of this Pull Request?

The Optimal Exposure Caclulator is a tool that is only used by a small number of people. As this was created when no plugin system was in place, there was no alternative.

Now however with the advancements in the code base, this makes much more sense to be put into a plugin.

The plugin is already prepared here: https://bitbucket.org/Isbeorn/nina.plugin.exposurecalculator/src/master/

How were the changes tested?

-

Did all Unit Tests pass?

Yes

Are there relevant Issues in the tracker that this PR will fix?

No

Screenshots

Notes