Address check problems

- [x] vignette has issue with pd matrix: Quitting from example.Rmd:296-310 [min-inf] <error/rlang_error> Error in `ModelParamsNormal()` at crmPack/R/Model-class.R:142:3: ! Assertion on 'h_is_positive_definite(cov, length(mean))' failed: Must be TRUE. - [x] -- re-building ‘knit_print.Rmd’ using rmarkdown Warning: Duplicated chunk option(s) 'label' in both chunk header and pipe comments of the chunk 'setup'. Warning: The vignette title specified in \VignetteIndexEntry{} is different from the title in the YAML metadata. The former is "demo", and the latter is "Describing `crmPack` Objects". If that is intentional, you may set options(rmarkdown.html_vignette.check_title = FALSE) to suppress this check.