prevent extract() working with numeric variables

[philmikejones]

extract creates a variable for each unique level of each variable in in the input individual-level data set (inds). If one of these variables is numeric it will almost certainly have a huge number of unique levels, especially for double().

Add a test to extract() so that it won't continue and output if it finds any numeric variables in the inds data set.

[philmikejones]

Closed in #50