Register the DataSource to use the nice fits alias

[JulienPeloton]

Now you can load FITS using the much nicer "fits" alias:

// full format name
val df = spark.read.format("com.astrolabsoftware.sparkfits").load(fn)

// With the alias
val df = spark.read.format("fits").load(fn)

[codecov-io]

Codecov Report

Merging #43 into master will increase coverage by 0.2%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master     #43     +/-   ##
========================================
+ Coverage   87.89%   88.1%   +0.2%     
========================================
  Files           9       9             
  Lines         479     479             
  Branches       83      83             
========================================
+ Hits          421     422      +1     
+ Misses         58      57      -1

Impacted Files	Coverage Δ
src/main/scala/com/sparkfits/DefaultSource.scala	100% <0%> (+33.33%)	⬆️

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