[FEAT] Exact reporting of missing values

[634523]

What aspect of the package is your request related to?

Reporting of missing values.

Describe the solution you'd like

Echoing #624 and @kylebutts in #624 I would also prefer if qsu() and descr() would show the exact number of NAs.

First, this would be consistent with base R summary(). Furthermore, it would avoid misleading cases like the example below:

library(collapse)

df <- data.frame(x = rnorm(1000000))
na_position <- sample(1:1000000, 1)
df$x[na_position] <- NA

descr(df)
qsu(df)

> descr(df)
Dataset: df, 1 Variables, N = 1000000
-----------------------------------------------------------------------------------------
x (numeric): 
Statistics (0% NAs)
       N   Ndist  Mean  SD    Min   Max  Skew  Kurt
  999999  999999    -0   1  -4.68  4.89    -0     3
Quantiles
     1%     5%    10%    25%  50%   75%   90%   95%   99%
  -2.33  -1.65  -1.28  -0.68   -0  0.67  1.28  1.64  2.32
-----------------------------------------------------------------------------------------
> qsu(df)
        N     Mean      SD      Min     Max
x  999999  -0.0024  0.9994  -4.6793  4.8876

Even though we have a missing value in the data.frame 0% NAs are reported.