Fixes rare issue with weight management regression

[eedrummer]

This fix addresses an issue where weight could be calculated as not a number
due to a divide by zero error.

This happened when a person had successful weight management, starting at age
13 and then regressing back to their original weight trajectory. The issue
would happen when attempting to determine what percentage of the "adult"
(after age 20) part of regression had elapsed. Since the adult regression
period was less than one year, it was setting the denominator of the
equation to zero.

The fix now sets this denominator to 1, making the calculation of percentage
elapsed the fraction over age 20 that the person is at the time of the weight
measurement.

Addresses issue #1038

There are checkstyle issues currently in the master branch that this PR does not address. Those issues are fixed in PR #1047

[codecov]

Codecov Report

Merging #1050 (d331e71) into master (7876535) will increase coverage by 1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master   #1050    +/-   ##
=========================================
+ Coverage        79%     80%    +1%     
- Complexity     3165    3222    +57     
=========================================
  Files           119     119            
  Lines         21704   21707     +3     
  Branches       2949    2950     +1     
=========================================
+ Hits          17168   17494   +326     
+ Misses         3617    3315   -302     
+ Partials        919     898    -21     

Impacted Files	Coverage Δ
...va/org/mitre/synthea/modules/WeightLossModule.java	99% <100%> (+<1%)	⬆️
.../world/agents/behaviors/ProviderFinderNearest.java	90% <0%> (-5%)	⬇️
...g/mitre/synthea/modules/HealthInsuranceModule.java	89% <0%> (-4%)	⬇️
...ain/java/org/mitre/synthea/world/agents/Payer.java	75% <0%> (-2%)	⬇️
.../main/java/org/mitre/synthea/engine/Generator.java	76% <0%> (-1%)	⬇️
...in/java/org/mitre/synthea/world/agents/Person.java	84% <0%> (-1%)	⬇️
...ava/org/mitre/synthea/modules/LifecycleModule.java	91% <0%> (-1%)	⬇️
src/main/java/org/mitre/synthea/engine/State.java	86% <0%> (+<1%)	⬆️
...c/main/java/org/mitre/synthea/export/Exporter.java	64% <0%> (+<1%)	⬆️
.../java/org/mitre/synthea/export/BB2RIFExporter.java	54% <0%> (+<1%)	⬆️
... and 12 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7876535...d331e71. Read the comment docs.