Pyranges

Additions:
- cluster and merge takes argument by to only merge cluster within specific features
- gr.features.introns added. Can use by="gene" or by="transcript"
- new data: pr.data.gencode_gtf and pr.data.ucsc_bed
- can subset pyrange with boolean vector
- sort also takes argument by (sort without arg sorts on start/end)

Fixes:
- bug in subset which removed strand
- bug when setting Strand with setattr
- bug when setting Chromosome with setattr

Changes:
- new method to compute cluster (3x as fast)
- string-arg to drop not interpreted as regex
- drop or keep do not take drop_strand. Only unstrand can drop strand.

Additions:
- subsetting with new col order rearranges columns

Changes:
- Now possible to reset Strand/Chromosome

Additions:
- gr.drop_duplicate_positions(strand=None) None means auto => true if stranded otherwise False
- add test data pr.data.chromsizes()
- pr.gf.tile_genome(genome_pyrange, tile_size, tile_last=False) (like GenomicRanges tileGenome)
- pr.gf.genome_bounds(pyrange, genome_pyrange, clip=False) (like UCSC bedclip)

Fixes:
- fix bug in tostring
- fix bug in multithreading

Additions:
- add apply_chunks, which operates on chunks, instead of chromosome-dfs.

Changes:
- add nb_cpu argument to all functions
- add number of columns and stranded/unstranded to tostring
- add ... as last column, when there are more columns than possible to show
- use , as thousands separator in tostring for number rows/cols

Additions:
- allow keyword-arguments to apply, apply_pair (see example in the docs)

Changes:
- to_csv etc. returns the objects themselves, so they can be used in method chains
- methods called tile/window instead of tiles/windows


Fixes:
- fix print when len(pr) < entries to print
- tile

Additions:
- add slack-flag to cluster/merge
- print joined positions possible
- add simple methods for printing without breaking the chain (p, mp, sp, tmp, rp)

Removals:
- settings in pyranges. Added print methods instead.

Improvement:
- print methods faster, especially for pyranges with many cols