more phase diagram fixes

[rkurchin]

• improve guessing by shifting by the previous delta instead of just reusing the result
• make tolerance for end of phase diagram a function argument
• figure out why I sometimes get T not defined error for find_phase_boundaries if I don't explicitly pass a temperature?
• maybe add some of the test coverage from better test coverage #45 ?

[codecov-commenter]

Codecov Report

Merging #53 (0efffc2) into main (ef8ed9a) will decrease coverage by 0.44%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
- Coverage   53.08%   52.64%   -0.45%     
==========================================
  Files          14       14              
  Lines         356      359       +3     
==========================================
  Hits          189      189              
- Misses        167      170       +3     

Impacted Files	Coverage Δ
src/phase_diagrams.jl	60.00% <0.00%> (-3.83%)	⬇️

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