Minor revisions to PR1027

This PR intends to simplify g.calibrate following this discussion in PR #1027:

windowsizes now hardcoded in g.calibrate to avoid need to check their length.
function g.downsample factored out as only a small part of it was being used, this part is now part of the g.calibrate code.
renamed a few object names in g.calibrate to make code more readable
tidied up code

Checklist before merging:

[ ] Existing tests still work (check by running the test suite, e.g. from RStudio).
[ ] Added tests (if you added functionality) or fixed existing test (if you fixed a bug).
[ ] Updated or expanded the documentation.
[ ] Updated release notes in inst/NEWS.Rd with a user-readable summary. Please, include references to relevant issues or PR discussions.
[ ] Added your name to the contributors lists in the DESCRIPTION file, if you think you made a significant contribution.

wadpac / GGIR