add a satpy example for custom composite by porting hr_overview from the mpop documentation

[peters77]

Is your feature request related to a problem? Please describe.
Please add an example to build a custom composite in satpy by porting examples from the mpop documentation (https://pytroll.readthedocs.io/en/latest/quickstart_seviri.html)

'hr_visual' ) - makes an enhanced black and white image from the HRV and

'hr_overview' - is a more complex composite using the HRV channel as luminance for the overview image

Describe the solution you'd like
An example in https://nbviewer.jupyter.org/github/pytroll/pytroll-examples/tree/master/satpy/ to show howto make a custom composite with enhanced MSG overview by HRV channel (luminance).

[pnuu]

PR #503 adds two compositors that can be used to to create luminance sharpened RGB composites. There are two built-in example composites to demonstrate these for SEVIRI: vis_sharpened_ir and ir_sandwich defined in satpy/etc/composites/seviri.yaml. This doesn't have documentation, but I'll be adding an issue about general documentation of composites and compositors.

[djhoese]

@pnuu Is this something that could still be useful? Maybe @gerritholl could add a wiki page or provide us with some information (I don't think it needs to go in satpy's main documentation) on converting composites from mpop to satpy? Or maybe fogpy's composites are too complicated to be made in to an example?