Rename `foo()`

[nanxstats]

There are two internal functions defined inline named foo():

gsDesign2/R/expected_time.R

Line 112 in 0f34f01

foo <- function(x) {

gsDesign2/R/gs_design_combo.R

Line 195 in 0f34f01

foo <- function(n, beta, ...) {

These functions should be updated:

1. Rename them to use a meaningful function name. See foo in Google's developer documentation style guide.
2. Define them outside the export function. This is because currently they are only accessible within the parent function, and it is being redefined again every time the parent function is called, which is suboptimal for performance.

[fb-elong]

How about the proposal below? If it is OK, you can assign the task to me.

1. get_ahr
2. get_power

[nanxstats]

1. Perhaps ahr_diff() communicates better that the function is for calculating the difference between the average hazard ratio and the targeted number of events, not for getting average hazard ratio per se.
2. Maybe get_combo_power()? Just to be consistent with the other existing internal function get_combo_weight().

[nanxstats]

Fixed by #229