Correct the Tau mass relative uncertainty.

[evgaster]

Tau mass constant had incorrect relative uncertainty.
It was based on Munon values.

Corrected the expression to use Tau values.

Signed-off-by: Ed@vanGasteren.net

[evgaster]

Again 2 commits that don't belong here pop up.
Despite the fact that I created a separate branche.
I tried to clear up the mess according to #1671 (comment) but that did not help this time.

[ppigazzini]

@evgaster :)

git fetch -p upstream
git switch issue-1681
# backup the branch
git switch -c issue-1681-bkp
git switch issue-1681
# get the hash of the last commit
git log -n 1
# reset to the branch you want to merge to
git reset --hard upstream/stable
# cherry pick the commit using its hash 
git cherry-pick 90cd6d2b4bb08f7d7879d3fde5c92277ec7a859b
# check the new history
git log -n 3
# if all is OK push to you remote (automatically to this PR)
git push -f origin HEAD
# delete the backup branch
git branch -D issue-1681-bkp

# if something went wrong, restore form the backup
git switch issue-1681
git reset --hard issue-1681-bkp

[evgaster]

@ppigazzini again, thank you very much. It not only fixes the problem but helps me better understand git (before I follow your instructions, I review the git docs to understand what I am about to do).

[ppigazzini]

IMO is really helpful to read the lean/elegant/simple git internals, git is simpler than expected
https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Porcelain

mastering the simple interactive rebase is really useful as well

git rebase -i upstream/stable

Create a branch backup until you will feel reckless and if something went wrong (huge conflict with many files) simply abort, same for cherry-pick

git rebase --abort
git cherry-pick --abort
``

[c3d]

This is on dev now, closing.