Fix tt2tdb() period and improve precision

[attipaci]

Another SuperNOVAS 1.0 oops. Beyond fixing the scaling tt2tdb() now uses the same series tdb2tt() series of the original NOVAS C.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.65%. Comparing base (20564a5) to head (212abc8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   98.65%   98.65%   -0.01%     
==========================================
  Files           3        3              
  Lines        2004     2003       -1     
  Branches      378      378              
==========================================
- Hits         1977     1976       -1     
  Misses          9        9              
  Partials       18       18              

Flag	Coverage Δ
unittests	98.65% <100.00%> (-0.01%)	⬇️

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Files	Coverage Δ
src/novas.c	98.44% <100.00%> (-0.01%)	⬇️

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