ENH: make list of accepted units overwritable/configurable

[petrrr]

This issue to propose a feature to overwrite the build in list of accepted units.

This might be an elegant way to address some of the other issues:

• It would allow to add units which are not present in the list, but are considered valid;
• If more restricted checking is desired (e.g. version 1.2, SI units only, base units only, ... ) the list could also be restricted by the user.

This would allow the behaviour of the validator to be adapted to individual needs and without the need to wait from new release. I also think that requirements might be conflictiong depending on usage. For example: relaxed checking for legacy metadata, strict checking against StationXML version 1.2 (or newer) for new submitted StationXML files.