`Execution` pattern update, additional 'abstract' properties

Following the agreement of this week's telecon, I have prepared a PR https://github.com/w3c/sdw-sosa-ssn/pull/226

This implements the following set of coupled changes

- move ObservingProcedure​ and SamplingProcedure​ into the sosa-oms:​ namespace used for additional terms required for the alignment of SSN and OMS
- add madeBySystem​ and madeExecution​ 'abstract' super-properties required for the generalized Execution pattern
- added an additional overview diagram for the Execution pattern https://raw.githack.com/w3c/sdw-sosa-ssn/225-madebysystem/ssn/index.html#overview-of-classes-and-properties
- added scope-notes explaining that System​, Execution​ and the new properties are to be considered 'abstract' and should not be used directly, e.g. https://raw.githack.com/w3c/sdw-sosa-ssn/225-madebysystem/ssn/index.html#SOSAExecution

I think these address the concerns raised in https://github.com/w3c/sdw-sosa-ssn/issues/225 and https://github.com/w3c/sdw-sosa-ssn/issues/235

I'd be grateful if you all would review the PR to check that I covered the bases.

Cheers - Simon

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