Coerce some integer data types

[amotl]

This is just a first shot at #107, further elaboration might touch or resolve aspects from #109.

[codecov]

Codecov Report

Merging #108 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   84.07%   84.19%   +0.11%     
==========================================
  Files          32       32              
  Lines         804      810       +6     
  Branches       85       87       +2     
==========================================
+ Hits          676      682       +6     
  Misses        103      103              
  Partials       25       25              

Flag	Coverage Δ
#unittests	84.19% <100.00%> (+0.11%)	⬆️

Impacted Files	Coverage Δ
wetterdienst/additionals/functions.py	84.61% <100.00%> (+1.08%)	⬆️

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[amotl]

Dear @gutzbenj,

thanks for improving the column type casting mentioned within #109 through your most recent contributions. I've rebased the branch of this PR on top of the current master branch.

Cheers,
Andreas.

[gutzbenj]

Dear Andreas,

I would rather integrate the integer types in the main function of type coercion. Also with the new column names class-enumeration-mixture arriving through #113 we should fix all those columns at once. I also put comments behind those columns which I believe are integers or strings.

All the best,
Benjamin

[amotl]

Dear Benjamin,

thanks for your excellent work on #113. I will rebase my branch on top of your improvements as soon as they have been merged and will follow your suggestions by integrating the integer type casting into the coerce_column_types() function.

With kind regards,
Andreas.

[gutzbenj]

Dear Andreas,

as we have already spoken about it, please also include those string data types in the type casting. Those rarely seen columns may hold information about the measuring type of e.g. cloud coverage.

[gutzbenj]

#115