factor out is_qdata() from check_data()
cosmetic
roxygenize
rename near_zero() to near_constant() and apply it to qscatter(), qmap() and qdensity()
fixed #197: define weight in qhist() meta as well
cosmetic
add 'handlers' in the common meta structure so users are able to add new event handlers later
new functions run_handler() and register_handlers() to deal with event handlers in meta
document 'handlers' and roxygenize