Move all control_* functions into one control function

[wwojciech]

There are few control functions currently existing in tern:

> packageVersion("tern")
[1] ‘0.7.4.9000’
> grep(pattern = "^control_", x = ls("package:tern"), value = TRUE)
[1] "control_coxph"          "control_coxreg"         "control_incidence_rate"
[4] "control_logistic"       "control_step"           "control_summarize_vars"
[7] "control_surv_time"      "control_surv_timepoint"

I think it would be elegant to consider grouping it into one control parametrised function. For instance, control(x = "coxph") would return what existing control_coxph returns.

[wwojciech]

@anajens - maybe this should go into Core Team queue as this is purely devops work?

[anajens]

I think it makes more sense for SME. Will move to backlog.