dilation size increased to 25

[palmito9]

Hi,
I have increased dilation size to max allowed in PixInsight: 25
I successfully tested it on PixInsight 1.8.9-1 for Windows

[beshanoe]

Hi, thanks for the contribution! One of these days I'll be able to take a look and release it

[palmito9]

Hi,

Great pleasure to contribute.

I have a second commit, let me know if you prefer I do only one PR with both or one PR per commit.

The second commit is for adding the possibility to choose an existing structure map.
In my case I needed to keep faint galaxies in the background and had to use a custom map.

I spoke about those two with Adam and he thinks they are very useful evolutions.

[beshanoe]

For another feature it's better to open a separate PR. Regarding this one, what do you think of including some sample images to this repo for the future testing? Unfortunately I don't have any material of the decent quality now.
Something like a samples folder with three images, original, starless and structures

[beshanoe]

Sorry I didn't read carefully enough. It's fine to keep structures choice in this PR as they are pretty closely related :)