Rename SurveyMap$new() arguments

[jgabry]

Right now the signature of SurveyMap$new is

SurveyMap$new(samp_obj, popn_obj, ...)

I think we should change it to

SurveyMap$new(sample, population, ...)

Several reasons:

• No need to use short names (samp, popn) given autocomplete (we already made this decision regarding method names
• No need for _obj suffix, it's redundant because they're obviously objects

Any objections?