FIX: rework the checks to account for accumulated problems

[ctrltz]

A bunch of issues with currently performed checks needs to be solved, potentially reconsidering the bigger picture of whether to support callables:

• BUG: check_coupling_params fails because of hard-coded sampling frequency #74: Duration and sampling frequency of the data is currently hard-coded in checks to have some value when testing callables - we should pass sfreq and duration directly to the SourceSimulator. Also, we can think of dropping the mechanism of callables, see another problem below
• BUG: empty patches are not handled properly #69: With callables, checks for user input are not enough - we need to double-check the result of every callable when it is called since the results of functions with randomly generated outputs might not always conform to our expectations
• Some of the checks have overlapping functionality, e.g. check_numeric_array and check_extents