Build small test data

[rcavalcante]

In inst/script build up BSseq-class objects with not too many loci and write them to bismark coverage format.

This data will be used to test:

• A filter function for min/max counts
• A filter function for C/T and G/A SNPs
• A tiling function based on genomic windows
• A tiling function based on given ranges
• A filter function based on minimum number of reads per group.

It will also be used to document how to read in data with bsseq::read.bismark in examples and the vignette.