Improve sits API, checks, and documentation to consider the recommendations of autotest

[gilbertocamara]

Describe the requested improvement
The autotest tool provided by ROpenSci uses the package documentation to test unstated assumptions on R functions. It tests how resilient a package is to modifications in the input parameters. Some of these tests are applicable to sits. Thus, the sits API is being improved based on the recommendations of autotest.

Associated sits API function
All functions where autotest finds problems.

[gilbertocamara]

After careful consideration and discussion with the autotest team, we consider to have sufficiently improved sits to match all relevant tests. There are some remaining issues indicated by autotest which we do not consider to be relevant.