Switch from excel files to yaml files as input source

[fmeynadier]

Following our discussions, commit 1892112 replaced the excel files used previously to generate the turtle files, by YAML files autogenerated from the xls. The YAML obtained this way is a simple conversion from the "table" format, so maybe not the smartest one, but at least it allows to track changes in the source. Adjusting them so they fit a schema could be the next step. Every sheet has been converted to a separate YAML file, so there are more YAML file than ther was excel files (we could also merge some of them, if required).

The attached diff.txt shows the difference in the ttl between the "main" branch just before merging, and just after. It includes fixes that have been ported in the meantime (uetta->zetta, fixes on xsd:double vs xsd:float).

To avoid ambiguity I removed the xls files from the main branch, but of course you can get them back by checking out 275ebf4