properly handle WCS when slicing Enmap

[xzackli]

This PR implements the proper WCS modifications one must make when slicing an enmap. This mostly involves modifying the CRPIX entry of the WCS structure. For example,

using Pixell

shape, wcs = fullsky_geometry(deg2rad(1); dims=(3,) )
m = Enmap(rand(shape...), wcs)
print(m.wcs.crpix, "\n", m[3:20, 50:70, 1].wcs.crpix)

[180.5, 91.0]
[178.5, 42.0]

If the stride is not 1, then CDELT can also change.

shape, wcs = fullsky_geometry(deg2rad(1))
m = Enmap(rand(shape...), wcs)
print(m.wcs.cdelt, "\n", m[3:5:24, 39:-3:2].wcs.cdelt)

[-1.0, 1.0]
[-5.0, -3.0]

In Julia, there is a syntactical distinction between slicing as copy and slicing as view. The usual array slice always copies! To avoid copies, you have to use @view or the view(...) function. This PR also provides the appropriate WCS manipulations when a view is made of an Enmap.

m = Enmap(rand(shape...), wcs)
m_view = @view m[3:24, 80:100]
print(m.wcs.crpix, "\n", m_view.wcs.crpix)

[180.5, 91.0]
[178.5, 12.0]