Add check for degrees of freedom to sdc_model() (#87)

[matthiasgomolka]

• Check implemented
• tests adapted accordingly
• documentation adapted accordingly

[codecov]

Codecov Report

Merging #98 (1e4f379) into development (6c27bca) will decrease coverage by 0.20%.
The diff coverage is 92.30%.

@@               Coverage Diff               @@
##           development      #98      +/-   ##
===============================================
- Coverage       100.00%   99.79%   -0.21%     
===============================================
  Files               12       12              
  Lines              468      481      +13     
===============================================
+ Hits               468      480      +12     
- Misses               0        1       +1     

Impacted Files	Coverage Δ
R/print_methods.R	98.96% <80.00%> (-1.04%)	⬇️
R/sdc_model.R	100.00% <100.00%> (ø)

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