Exclude message for r2, r2adj, corrxy when half life points are less than 3

[Gero1999]

From the related issue discussion in #503, we figured out that it could be nice to either:

• change the default exclude mechanism to allow making exclusions only for particular parameter outputs in the half.life() calculations, rather than one homogenous exclude message for all

• otherwise create a exclude function to populate the message 2 points used for half-life calculation for r.squared, adj.r.squared and corrxy with half.life_points < 3

2 points used for half-life calculation should not be also a exclude message for r.squared, adj.r.squared rather than just a warning? It will make it more CDISC reportable and specific to the profile. Right now, this is what happens:

df <- Theoph %>%
  filter(Subject == 1) %>%
  filter(Time %in% c(2.02, 7.03, 12.12))
o_conc <- PKNCAconc(df, formula = "conc ~ Time | Subject")
o_data <- PKNCAdata(data.conc = o_conc, intervals = data.frame(lambda.z = TRUE, start = 0, end = Inf))
PKNCA.options(min.hl.points = 2)
as.data.frame(pk.nca(o_data))

No dose information provided, calculations requiring dose will return NA.
# A tibble: 12 × 7
   Subject start   end PPTESTCD            PPORRES PPANMETH exclude
   <ord>   <dbl> <dbl> <chr>                 <dbl> <chr>    <chr>  
 1 1           0   Inf tmax                 2.02   ""       NA     
 2 1           0   Inf tlast               12.1    ""       NA     
 3 1           0   Inf lambda.z             0.0450 ""       NA     
 4 1           0   Inf r.squared            1      ""       NA     
 5 1           0   Inf adj.r.squared       NA      ""       NA     
 6 1           0   Inf lambda.z.corrxy     -1      ""       NA     
 7 1           0   Inf lambda.z.time.first  7.03   ""       NA     
 8 1           0   Inf lambda.z.time.last  12.1    ""       NA     
 9 1           0   Inf lambda.z.n.points    2      ""       NA     
10 1           0   Inf clast.pred           5.94   ""       NA     
11 1           0   Inf half.life           15.4    ""       NA     
12 1           0   Inf span.ratio           0.331  ""       NA     
Warning messages:
1: n must be > 2 for adj.r.squared 
2: 2 points used for half-life calculation 

For me personally does not sound right to consider an r.squared of 2 points for any summary table, but what do you think?

Originally posted by @Gero1999 in #503