Issue 76: preserve column order from the columns option

[JulienPeloton]

This PR fixes the wrong column ordering when the option columns was used:

df = spark.read.format("fits")\
  .option("hdu", 1)\
  .load("src/test/resources/test_file.fits")

df.show(2)
# +----------+--------+-------------------+-----+-----+
# |    target|      RA|                Dec|Index|RunId|
# +----------+--------+-------------------+-----+-----+
# |NGC0000000|3.448297|-0.3387486324784641|    0|    1|
# |NGC0000001|4.493667|-1.4414990980543227|    1|    1|
# +----------+--------+-------------------+-----+-----+

df = spark.read.format("fits")\
  .option("hdu", 1)\
  .option("columns", "Index,target,RunId")
  .load("src/test/resources/test_file.fits")

df.show(2)
# +-----+----------+-----+
# |Index|    target|RunId|
# +-----+----------+-----+
# |    0|NGC0000000|    1|
# |    1|NGC0000001|    1|
# +-----+----------+-----+

[codecov-io]

Codecov Report

Merging #77 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   90.22%   90.24%   +0.01%     
==========================================
  Files           9        9              
  Lines         532      533       +1     
  Branches       99      101       +2     
==========================================
+ Hits          480      481       +1     
  Misses         52       52

Impacted Files	Coverage Δ
...m/astrolabsoftware/sparkfits/FitsHduBintable.scala	93.05% <100%> (+0.09%)	⬆️

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