Implement code generation

[dominformant]

With this pull request, the generation of codes based on sampling rate (and optionally code rate and start phase) is implemented in GNSSSignals. This allows to un-export get_code_unsafe, which leads to errors in the generation of BOC-Codes.

[codecov-commenter]

Codecov Report

Merging #21 (143ac2e) into master (4600806) will increase coverage by 4.71%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   73.18%   77.90%   +4.71%     
==========================================
  Files           6        6              
  Lines         179      181       +2     
==========================================
+ Hits          131      141      +10     
+ Misses         48       40       -8     

Impacted Files	Coverage Δ
src/GNSSSignals.jl	71.42% <ø> (-6.35%)	⬇️
src/boc.jl	93.54% <ø> (-0.40%)	⬇️
src/common.jl	61.70% <88.88%> (+25.11%)	⬆️
src/galileo_e1b.jl	68.18% <100.00%> (ø)
src/gps_l1.jl	90.90% <100.00%> (ø)
src/gps_l5.jl	82.69% <100.00%> (ø)

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