Script generates too many milestones?

[rcannood]

With end_n == 2

library(dyno)
library(tidyverse)
data("fibroblast_reprogramming_treutlein")
dataset <- wrap_expression(
  counts = fibroblast_reprogramming_treutlein$counts,
  expression = fibroblast_reprogramming_treutlein$expression
)
model <- infer_trajectory(dataset = dataset %>% add_prior_information(end_n = 2), method = ti_grandprix(), verbose = TRUE)
model$milestone_network

# A tibble: 2 x 4
  from            to        length directed
  <chr>           <chr>      <dbl> <lgl>   
1 milestone_begin split_0_1      1 TRUE    
2 milestone_begin split_0_2      1 TRUE    

With end_n == 3

model <- infer_trajectory(dataset = dataset %>% add_prior_information(end_n = 2), method = ti_grandprix(), verbose = TRUE)
model$milestone_network

# A tibble: 6 x 4
  from            to        length directed
  <chr>           <chr>      <dbl> <lgl>   
1 milestone_begin split_0_1      1 TRUE    
2 milestone_begin split_0_2      1 TRUE    
3 milestone_begin split_1_1      1 TRUE    
4 milestone_begin split_1_2      1 TRUE    
5 milestone_begin split_2_1      1 TRUE    
6 milestone_begin split_2_2      1 TRUE    

Also, when end_n == 1, a more useful error message should be generated (See dynverse/dyno#30)