Issue 71: better control of recordLength value, and related error

[JulienPeloton]

Fix issue #71, by throwing an error if one specifies recordLength < rowSize, and adapting recordLength otherwise (default is recordLength = 1024 Bytes).

Let suppose we have a file with row size 2 KB (> 1 KB).

Case 1: default behaviour

# will work (was not working previously)
# The recordLength is equal to row size.
df = spark.read.format("fits")\
  .option("hdu", 1)\
  .load("path/to/my.fits")

Case 2: specifying a large recordLength

# will work (was working previously)
# The recordLength is equal to 5 KB.
df = spark.read.format("fits")\
  .option("hdu", 1)\
  .option("recordlength", 5*1024)\
  .load("path/to/my.fits")

Case 3: specifying a too small recordLength

# will throw an error (was throwing a cryptic error previously)
df = spark.read.format("fits")\
  .option("hdu", 1)\
  .option("recordlength", 1024)\
  .load("path/to/my.fits")
...
Py4JJavaError: An error occurred while calling o35.load.
: java.lang.AssertionError:
          You specified a recordLength option too small compared to the FITS row size:
          1024 B < 2048 B
	at com.astrolabsoftware.sparkfits.FitsLib$Fits.<init>(FitsLib.scala:196)
...

[codecov-io]

Codecov Report

Merging #72 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   90.11%   90.22%   +0.11%     
==========================================
  Files           9        9              
  Lines         526      532       +6     
  Branches       99       99              
==========================================
+ Hits          474      480       +6     
  Misses         52       52

Impacted Files	Coverage Δ
.../astrolabsoftware/sparkfits/FitsRecordReader.scala	75.7% <100%> (ø)	⬆️
...scala/com/astrolabsoftware/sparkfits/FitsLib.scala	97.51% <100%> (+0.09%)	⬆️

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