Improvement to iso_schematron_skeleton_for_saxon.xsl - param instead of variable

[phax]

To have the possibility to pass in certain values from the outside, axsl:param gives more flexibility compared to axsl:variable.
This is based on phax/ph-schematron#8 (comment)

I changed it e.g. here: https://github.com/phax/ph-schematron/blob/master/ph-schematron/src/main/resources/schematron/20100414-xslt2/iso_schematron_skeleton_for_saxon.xsl#L1337-L1339

As I'm not an XSLT expert there might be good reasons why it is like it is, but on the other hand, opening for data from the outside might be valuable.