make _get_slices a public function #29

Currently, the name of array_utils._get_slices indicates that this is a private function and should not be used by the user.
Why is that so?

I propose to expose this function publically. There are two functions that use _get_slices, but they are essentially very thin wrappers around the _get_slices functionality.
They allow to extract or add (add_array_2d and extract_array_2d) using the slices from _get_slices. However, there are other uses for _get_slices:

• assign a value to a subregion (e.g. to mask it)
• use the slices to address a related array of same size as the data array (e.g. a mask array)
• extract the overlapping part of a 2d array, even if it only partially overlaps (extract_array_2d will raise an Exception in this case.)

I have use cases for all three options in PSF phometry with masked values. I need to keep a mask in snyc with the data (my masks are calculated and do not come with the data, so it is more logical to use them separately instead of an NDData object, even when that works) and I need to do PSF photometry, even if the PSF only partially overlaps with the image.

We could either write wrappers for those uses, too, or we could just rename _get_slices to get_slices.