Design check

[mb706]

Closes #268

This is a bit of an "opinionated" PR:

• checking a df against a ParamSet could have been done with a new function (either member of ParamSet or global), but instead the $check() is polymorphic now. This gives us $assert and $test for free. Note I had to split up $check for the common stuff between data.frame and list to avoid unnecessary repeated checks of parameter names in the data.frame case
• Design$new() now checks for feasibility of its argument, because implicitly users of this were assuming that it does (principle of least surprise)
• Design$new() used to "enforce" dependencies; this is optional now and does not happen by default (see point above). This may break one or two users somewhere that relying on default enforcement of deps.
• Design$new() parameter remove_dupl defaults to FALSE now because that is obviously the right choice for a default here.

If there are small problems with this please fix or delegate to a hiwicontributor.

[codecov-io]

Codecov Report

Merging #270 into master will decrease coverage by 0.08%.
The diff coverage is 97.29%.

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
- Coverage   95.16%   95.07%   -0.09%     
==========================================
  Files          20       20              
  Lines         496      508      +12     
==========================================
+ Hits          472      483      +11     
- Misses         24       25       +1

Impacted Files	Coverage Δ
R/Sampler.R	87.5% <100%> (ø)	⬆️
R/generate_design_grid.R	91.3% <100%> (ø)	⬆️
R/generate_design_lhs.R	100% <100%> (ø)	⬆️
R/Design.R	94.59% <100%> (+0.15%)	⬆️
R/ParamSet.R	96.83% <96.87%> (-0.45%)	⬇️

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[berndbischl]

@mb706 @be-marc
apparently we should all communicate better.
marc already did the check_dt function.

i will keep the simpler solution and close this for now