SimpleUnitFormat.parse() does not always work with prefixes

[keilw]

There are at least one or two cases where SimpleUnitFormat.parse() (or AbstractUnit.parse()) won't recognize a string that can be properly formatted, it seems related to prefixes where observed.

E.g. AbstractUnit.parse("dm") works just the same way as DECI(METRE).toString(), but the result of
DECI(CARAT).toString() fails parsing it with AbstractUnit.parse("dct").
Note CARAT is defined by the extension module Unicode CLDR, but it should not really matter if the unit is defined by the RI itself or an extension.

[cmmercer]

I have also encountered this when using a custom unit (of geologic time, the "annus" with symbol "a") that I defined in my own SystemOfUnits. I took care to register a label with SimpleUnitFormat, so I can parse it when the unit does not have a prefix. But when I instantiate the unit with a metric prefix and try to parse the result of calling toString() (e.g., "Ma" or "ka") it fails.

[keilw]

I know, this one has been around for some time. For certain units, they are not even meant to work with prefixes, see #433.
We faced that problem trying to support all prefixes for units like YEAR (also using the symbol "a") and PETA(YEAR) would then result in "Pa" which is also the standard symbol for PASCAL.

We could in a future version (probably not before 2.2.4 or later) consider making aliasWithPrefixes() and labelWithPrefixes() both currently private to the default subclass of SimpleUnitFormat public, similar to removeAlias() or removeLabel(). Both of which may be used to clean or change a custom prefix definitionlike the ones mentioned.

I think we should discuss this a bit further in the discussion forum before making those methods visible.