Move npc geoms to their own package

[Pedro J. Aphalo]

An initial bad design decision means that both packages export statistics for peaks and valleys with the same name. The code is similar, but incompatible. It is now clear that several geoms defined in ‘ggpmisc' would be very useful in ‘ggspectra’ I need to work out a way to solve this, possibly splitting ‘ggpmisc’ once again into two packages, moving the ‘npc’ geoms to their own package, with a name like 'ggnpc'.

For the time being, we could import from ‘ggpmisc’ into ‘ggspectra’ but the imports would break later.

[Pedro J. Aphalo]

Increasing the number of packages is not a very good idea. I will keep packages as they are and selectively import and re-export functions from ggpmisc in ggspectra.

[Pedro J. Aphalo]

There are better ways to get around this problem based on selective imports.