Add some warnings and errors for cohort methods.

[syroBx]

Add warning for 'update_filter' and 'stat' function for #15

update_filter

Before:

> coh$update_filter(1L, "film_filter", active = "FALSE")

> get_state(coh)
[[1]]
[[1]]$step
[1] "1"
...
[[1]]$filters[[2]]$active
[1] "FALSE"

> run(coh)
Error in `purrr::keep()`:
ℹ In index: 2.
ℹ With name: film_filter.
Caused by error:
! `.p()` must return a single `TRUE` or `FALSE`, not a string.
Run `rlang::last_trace()` to see where the error occurred.

After:

> coh$update_filter(1L, "film_filter", active = "FALSE")
Warning message:
In coh$update_filter(1L, "film_filter", active = "FALSE") :
  Active accepts only logical values

> get_state(coh)
[[1]]
[[1]]$step
[1] "1"
...
[[1]]$filters[[2]]$active
[1] TRUE

> run(coh)

> coh$update_filter(1L, "film_filter", active = FALSE)
> get_state(coh)
[[1]]
[[1]]$step
[1] "1"
...
[[1]]$filters[[2]]$active
[1] FALSE

stat

Before:

> stat(coh,10)
$actor
$actor$n_rows
NULL

...

$store
$store$n_rows
NULL

After:

> stat(coh,10)
Error in x$get_stats(step_id, filter_id, ..., state = state) : 
  Step is not exist in this cohort object.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.51%. Comparing base (086713f) to head (315f4d9).

Files with missing lines	Patch %	Lines
R/cohort_methods.R	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #80      +/-   ##
==========================================
+ Coverage   66.41%   66.51%   +0.10%     
==========================================
  Files          11       11              
  Lines        1977     1983       +6     
==========================================
+ Hits         1313     1319       +6     
  Misses        664      664              

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