Improve doc for examples of controlling reading/writing

[neilflood]

Also some minor changes to docstrings in support of this.

[gillins]

ping @t-hackwood

[gillins]

Thanks @neilflood - I think that looks better. I wonder if we should also mention that pixel values from outside an image will be 'faked' with the nodata value for that image (for eg. UNION)? And what do we do if there is no nodata (fail)?

[neilflood]

Good points, thanks @gillins

I expanded the docstring for setFootprintType, so this information is not just in the examples doco. I had to look up about the "no null value" case, it is in imagereader.py, line 541, readBlockWithMargin(). The logic is really there for the normal margin handling anyway, but the UNION case effectively makes huge margins on some inputs.

[gillins]

Thanks @neilflood looks good to me!

[t-hackwood]

Thanks for the updated examples @neilflood @gillins! They look great, much easier to understand!

[neilflood]

OK, great, thank you. I will merge this :-)

And thanks for bringing this to our attention.