Correcting the standard uncertainty unit for several constants

[evgaster]

"rad", "twoπ", "angl", "UnixEpoch", "SinclairZX81RAM" and "PageSize"
returned a standard uncertainty without a unit, while the value does have a unit.

"MC12" and "ε₀" returned a standard uncertainty with a unit different from the unit of the value.

Corrected the standard uncertainty unit for those constants.

Signed-off-by: Ed@vanGasteren.net

[evgaster]

The commit about the Tau mass doesn't belong in this PR. It wont do any harm, but still it doesn't belong here.
In previous PRs I ended up with several of those commits. I seem to be making some progress in getting rid of them. Still one to go :-).

[ppigazzini]

@evgaster :)

git fetch -p upstream
git switch issue-1683
# backup the branch
git switch -c issue-1683-bkp
git switch issue-1683
# get the hash of the last commit, now we get the short one, it's the same that you see in the PR
git log -n 1 --oneline
# reset to the branch you want to merge to
git reset --hard upstream/stable
# cherry pick the commit using its hash 
git cherry-pick 1abc966
# check the new (oneline) history
git log -n 3 --oneline
# 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-1683-bkp

# if something went wrong, restore form the backup
git switch issue-1683
git reset --hard issue-1683-bkp

[evgaster]

See #1682 (comment) :-)

[c3d]

This has been pushed on dev, closing.