Add Earth position vector

[aarchiba]

Closes #903

[codecov]

Codecov Report

Merging #944 (caafcce) into master (0ba02fa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #944   +/-   ##
=======================================
  Coverage   55.61%   55.61%           
=======================================
  Files          85       85           
  Lines       17733    17733           
  Branches     2999     2999           
=======================================
  Hits         9863     9863           
  Misses       7245     7245           
  Partials      625      625           

Impacted Files	Coverage Δ
src/pint/toa.py	83.42% <100.00%> (ø)

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