Issue 78: 1X should be viewed as scalar

[JulienPeloton]

Following #78, this PR introduces new change to convert 1 element vector from FITS (1X) to a scalar in Spark (XType) and not array of one element:

# Example
 E -> float -> FloatType
1E -> float -> FloatType
nE -> array of n floats -> ArrayType(FloatType)

[codecov-io]

Codecov Report

Merging #79 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage   90.24%   90.51%   +0.26%     
==========================================
  Files           9        9              
  Lines         533      548      +15     
  Branches      101       97       -4     
==========================================
+ Hits          481      496      +15     
  Misses         52       52

Impacted Files	Coverage Δ
...scala/com/astrolabsoftware/sparkfits/FitsLib.scala	97.51% <100%> (ø)	⬆️
...la/com/astrolabsoftware/sparkfits/FitsSchema.scala	85.29% <100%> (+2.53%)	⬆️
...scala/com/astrolabsoftware/sparkfits/FitsHdu.scala	95% <100%> (+1%)	⬆️

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