Add some constants

[jorgepiloto]

Partially solves for #46 by adding Earth's radius and gravity acceleration surface value. Makes use of the units module ✈️

[codecov-io]

Codecov Report

Merging #48 into master will decrease coverage by 0.66%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   72.78%   72.12%   -0.67%     
==========================================
  Files          10       11       +1     
  Lines         327      330       +3     
==========================================
  Hits          238      238              
- Misses         89       92       +3

Impacted Files	Coverage Δ
src/skaero/constants.py	0% <0%> (ø)

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[jorgepiloto]

Though decreasing the coverage, I will merge this since does not introduce huge complexity in the code. Furthermore, let me open an issue on coverage increase for the overall package.