improve altitude bin validation read_vpts() and as.vpts()

[adokter]

this PR adds checks for the number of altitude bins and their spacing, and retains profiles only with the most common altitude bin definition (number of bins and bin size). Since current vpts objects can only handle one type of altitude profile, only the most common type is retained.

[codecov]

Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.35%. Comparing base (d1c6a8e) to head (5fdf406).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
R/as.vpts.R	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #684      +/-   ##
==========================================
+ Coverage   81.87%   82.35%   +0.47%     
==========================================
  Files          61       61              
  Lines        3703     3717      +14     
==========================================
+ Hits         3032     3061      +29     
+ Misses        671      656      -15     

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