two-node model iteration

[FedericoTartarini]

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Describe the bug

1. The code is not efficient since the main loops runs for 60 times:
   1. this is unnecessary in most conditions since the results converge before the 60 iterations, however,
   2. for some inputs, 60 iterations are not sufficient and terminate the code routine before the code has converged.

To Reproduce

Example 1

two_nodes(tdb = 45,tr = 45,v = 1.1,rh = 20,clo = 0.2,met = 3)

Example 2

(tdb=25, tr=25, v=0.6, rh=50, met=1, clo=.5)

Expected behavior

We should stop the iteration only when the output converges.