Include new sampled data example

[jakobdambon]

Since the RandomFields has been archived on CRAN, we want to provide a new sampled data example. We hope that in the future, the RandomFields package will be available on CRAN such that we can use the v.0.3.1 functions to sample data.

[jakobdambon]

Check out spam::rmvnorm function suite (and potentially migrate) to provide a similar experience to v.0.3.1 functions fullSVC_reggrid, fullSVC_line, or sample_fullSVC.

[jakobdambon]

Dropping old fullSVC_reggrid and fullSVC_line functions as they can be easily replicated by using sample_fullSVC. Simply sample /determine the locations beforehand. Renaming the sample_fullSVC as one can also sample data from a non-full SVC model, i.e., a model with covariates that have

1. a fixed and a random effect
2. only a random effect
3. only a fixed effect
4. neither a fixed nor random effect.

Added unit testing for example.