SeqFileIn's readRecords() can load IUPAC Fastq files into e.g. Dna5 StringSets

I added an optional tag to readRecords() to control the alphabet of the input file.

This tag is not strictly necessary for the single readRecord(). In this case, the conversion can be controlled by passing a Iupac target String to readRecord() and then assigning manually an Iupac String to eg. Dna5 String.

... I meant to remove the `maxRecords` default as well.