Mpes app

[rettigl]

Development branch for NXmpes app
See #37

[lukaspie]

@rettigl thanks for starting this. We should think how/where we want to ship this. If we have it as part of pynxtools-mpes, we will not have the app when we only use pynxtools-xps without the mpes package.

I see different ways we could go:

1. we require pynxtools-mpes within the dependencies of pynxtools-xps so the app always gets installed
2. we make a separate repo just for the app (like pynxtools-mpes-app and add this here and in ´pynxtools-xps´ in the deps)
3. we merge pynxtools-mpes and pynxtools-xps again into one repository/package

What are your opinions on this?

[rettigl]

Maybe it indeed would make sense to have also two separate apps. E.g. searching for specific core level energies or so is interesting for NXxps, but for NXmpes not so much...
Difficulty is to keep the part of development that is shared synced.