allow to find individual components Scotch and PTScotch

Summary

This is a small improvement of the existing FindPTScotch cmake module. It allows to search only for Scotch or PTScotch and thus it is possible to use Scotch without PTScotch or if no MPI is available.

added 1 commit

• f33b4e1e - set more required vars

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changed the description

added buildsystem label

@simon.praetorius , no Scotch user seems to object, so please go ahead and merge this.

merged

mentioned in commit 630f5f0f

mentioned in issue #199 (closed)