add_* functions

[adibender]

• add_hazard, add_cumuhazard, add_term functions somewhat clunky
• also a lot of redundancy
• need to add functions that check that variables allready present are not overwritten

[adibender]

could rewrite all of these as augment functions (broom style). Type of augmentation - hazard, cumulative haazard, survivor, etc. - could be provided as argument