pv: adding xavp_copy with destination index

[jchavanton]

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

MR to add another xavp_copy function taking 4 params

 xavc_copy("source_name", "source_index", "destination_name", "destination_index")

This will let the user copy to a specific position in the stack (replacing its value).
This would bring coherent behaviour compared to assign to a specific index vs appending.

Example : swapping xavp in and existing stack:
(second and third elements)

$xavp(a=>x) = "a-0-x";
$xavp(a[0]=>y) = "a-0-y";
$xavp(a=>x) = "a-1-x";
$xavp(a[0]=>y) = "a-1-y";
$xavp(a=>x) = "a-2-x";
$xavp(a[0]=>y) = "a-2-y";

# INFO: <script>: NEW $xavp(a[0]) = [a-2-x][a-2-y]
# INFO: <script>: NEW $xavp(a[1]) = [a-1-x][a-1-y]
# INFO: <script>: NEW $xavp(a[2]) = [a-0-x][a-0-y]

xavp_copy("a", "1", "c");
xavp_copy("a", "2", "a", "1");
xavp_copy("c", "0", "a", "2");

# INFO: <script>: AFTER $xavp(a[0]) = [a-2-x][a-2-y]
# INFO: <script>: AFTER $xavp(a[1]) = [a-0-x][a-0-y]
# INFO: <script>: AFTER $xavp(a[2]) = [a-1-x][a-1-y]

[jchavanton]

I will update the documentation.
I will probably do another round of testing.