Fixes for issues found by SpotBugs

[jmthibault79]

Description

Resolves #1279 - removed the ExternalBlock class which was not truly necessary Resolves #1280 - removed the unused CramIterator.nextRecord field Resolves #1281 - removed the unused CramHeader.clone() method

Checklist

• [x] Code compiles correctly
• [x] New tests covering changes and new functionality
• [x] All tests passing
• [x] Extended the README / documentation, if necessary
• [ ] Is not backward compatible (breaks binary or source compatibility)

[codecov-io]

Codecov Report

Merging #1283 into master will increase coverage by 0.006%. The diff coverage is 78.571%.

@@               Coverage Diff               @@
##              master     #1283       +/-   ##
===============================================
+ Coverage     67.495%   67.501%   +0.006%     
+ Complexity      8150      8149        -1     
===============================================
  Files            558       557        -1     
  Lines          33364     33352       -12     
  Branches        5608      5607        -1     
===============================================
- Hits           22519     22513        -6     
+ Misses          8657      8651        -6     
  Partials        2188      2188

Impacted Files	Coverage Δ	Complexity Δ
...ava/htsjdk/samtools/cram/structure/CramHeader.java	48% <ø> (+8%)	7 <0> (ø)	:arrow_down:
src/main/java/htsjdk/samtools/CRAMIterator.java	81.343% <0%> (+0.184%)	32 <0> (ø)	:arrow_down:
...ain/java/htsjdk/samtools/cram/structure/Slice.java	60.248% <100%> (ø)	38 <0> (ø)	:arrow_down:
...va/htsjdk/samtools/cram/structure/block/Block.java	75.362% <83.333%> (+0.362%)	23 <7> (+1)	:arrow_up: