added custom mipmap chains
ctr for MipMapChain
drip in a getter lens
docs an reexporting
switched to `semigroup`s `NonEmpty` from `mono-traversable`'s `NotNull`

`NonEmpty` is easier and more idiomatic to map and to traverse
because it's not a mono-traversable.
moved gl-error check to the `Yage.Core.OpenGL` module

gl-error check throws now a exception
added mipmap level count to texture show
added `MipMapChain` to reexport
additional utility functions
added cubemap from list function