[feature] Expose stencil vertical domain to Frozen Stencil

[FlorianDeconinck]

Description
The new feature in development to allow absolute indexing in K allows to easily code patterns like "get the top of the column"... as long as you know what top is.

As we have done in the past for I and J, we expose via __externals__ the bounds of K for the stencil.

This is a zero-cost variable (no cost when not used, no perf cost when used)

How Has This Been Tested?
Extended utest

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[twicki]

LGTM

[bensonr]

The logic looks good.