Write some more tests

[hanneoberman]

[hanneoberman]

Current code coverage = 82%. Please feel free to add some use cases!

[hanneoberman]

For example, test whether plot_pred() works if blocks are defined

[gerkovink]

Will do

[hanneoberman]

Test if all functions work when the data has variable names that are used internally as well. For example, .where is used for the missingness indicator, but what if it's a variable name too?

[gerkovink]

Also .id or .imp as a column name.