Handling SI Digital Reference units of legacy realizations

[ronaldtse]

Originally from #111

There are additional SI units that we don't have, which are legacy realizations of the base units. I wonder if we to incorporate them in UnitsDB (probably).

Found 20 differences in units:
+---------------------------------------------------------------+
|                     Unit Name Differences                     |
+-----------------+--------------------+------------------------+
| Identifier      | SI Reference (TTL) | YAML                   |
+-----------------+--------------------+------------------------+
| SI#ampere1948   | ampere1948         | (not found)            |
| SI#ampere2018   | ampere2018         | (not found)            |
| SI#candela1948  | candela1948        | (not found)            |
| SI#candela1967  | candela1967        | (not found)            |
| SI#candela1979  | candela1979        | (not found)            |
| SI#candela2018  | candela2018        | (not found)            |
| SI#kelvin1967   | kelvin1967         | (not found)            |
| SI#kelvin2018   | kelvin2018         | (not found)            |
| SI#kilogram1901 | kilogram1901       | (not found)            |
| SI#kilogram2018 | kilogram2018       | (not found)            |
| SI#metre1889    | metre1889          | (not found)            |
| SI#metre1927    | metre1927          | (not found)            |
| SI#metre1960    | metre1960          | (not found)            |
| SI#metre1983    | metre1983          | (not found)            |
| SI#metre2018    | metre2018          | (not found)            |
| SI#mole1971     | mole1971           | (not found)            |
| SI#mole2018     | mole2018           | (not found)            |
| SI#second1960   | second1960         | (not found)            |
| SI#second1967   | second1967         | (not found)            |
| SI#second2018   | second2018         | (not found)            |
+-----------------+--------------------+------------------------+