We need a section in InaSAFE docs explaining how clipping works

[timlinux]

We are busy working to implement logic like this:

The following logic will be used:

• The clip extent of the northern edge will be decremented until it aligns with a pixel boundary of the target layer. If the target layer's northern edge is interior to the extents, the northern edge will be incremented until it aligns with a pixel boundary.
• The clip extent of the southern edge will be incremented until it aligns with a pixel boundary of the target layer. If the target layer's southern edge is interior to the extents, the southern edge will be decremented until it aligns with a pixel boundary.
• The clip extent of the eastern edge will be decremented until it aligns with a pixel boundary of the target layer. If the target layer's eastern edge is interior to the extents, the eastern edge will be incremented until it aligns with a pixel boundary.
• The clip extent of the western edge will be incremented until it aligns with a pixel boundary of the target layer. If the target layer's western edge is interior to the extents, the western edge will be decremented until it aligns with a pixel boundary.
  You can clone / edit the above document here:
  https://www.lucidchart.com/documents/edit/379424d4-8c44-4ece-88cc-8792a7e8274b/0

There are various other pieces of logic relating to clipping and alignment that should be documented so that users can understand what happens under the hood.

@akbargumbira and @assefay Should provide their inputs on this too.

[akbargumbira]

@timlinux this is great. Perhaps we should also explain from the very first:

• How we get the optimal extent from input layers and the viewport
• How we adjust that optimal extent to align with one of the input layer (either hazard/exposure)
• That we can also allow InaSAFE not to resample exposure layer (if it's raster and the hazard has higher resolution - as it will resample both to the highest resolution)
• How we do the clipping/resampling based on the adjusted extent

[timlinux]

Yes and

• How extent is also the intersection of the hazard and exposure and analysis area
• How the user can specify a custom analysis area or the current map extents are used (see We need to allow the user to load and save analysis extents inasafe#1288)

[timlinux]

See also #185