Generate valid mix10 and mix20 metadata (various values are Rational).

[carlwilson]

The Brightness, ApertureBias and FlashEnergy properties are now Rational as needed for the versions 1.0 and 2.0 of the MIX schema.
Add unit test for all the new outputs.
Should also correct #387 : need for a sample file.

[codecov]

Codecov Report

Merging #429 into rc/jhove-1.22 will increase coverage by 4.03%.
The diff coverage is 70.96%.

@@                 Coverage Diff                 @@
##             rc/jhove-1.22     #429      +/-   ##
===================================================
+ Coverage            48.67%   52.71%   +4.03%     
- Complexity             954     1119     +165     
===================================================
  Files                   55       55              
  Lines                 7659     9343    +1684     
  Branches              1393     2284     +891     
===================================================
+ Hits                  3728     4925    +1197     
- Misses                3458     3890     +432     
- Partials               473      528      +55

Impacted Files	Coverage Δ	Complexity Δ
...edu/harvard/hul/ois/jhove/handler/TextHandler.java	0.64% <0%> (+0.09%)	4 <0> (+2)	⬆️
.../edu/harvard/hul/ois/jhove/handler/XmlHandler.java	71.11% <100%> (+10.11%)	354 <0> (+98)	⬆️
...a/edu/harvard/hul/ois/jhove/NisoImageMetadata.java	74.64% <100%> (+0.86%)	175 <0> (+2)	⬆️
...main/java/edu/harvard/hul/ois/jhove/JhoveBase.java	48.65% <0%> (+5.79%)	78% <0%> (+36%)	⬆️
.../java/edu/harvard/hul/ois/jhove/ConfigHandler.java	69.23% <0%> (+9.23%)	50% <0%> (+24%)	⬆️
...edu/harvard/hul/ois/jhove/ChecksumInputStream.java	78.94% <0%> (+10.52%)	10% <0%> (+1%)	⬆️
...in/java/edu/harvard/hul/ois/jhove/Checksummer.java	79.66% <0%> (+11.86%)	15% <0%> (+2%)	⬆️

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