Pyranges

- add Simes method to pr.stats
- add keys argument to pr.iter
- make strand=None default arg for concat
- gr.split() does opposite of merge
- pr.count_overlaps(grs, features=None) like bedtools multiintersect added
- set mkl.set_num_threads to 1 in __init__

- add value col argument to to_bigwig (thanks https://github.com/liyao001)

- fix regression: slack changes dtype from int32 to int64

- add dtypes attribute to pyranges
- fix left and right join when chromosomes missing

- add argument sparse to read_bam. Setting it to False fetches more columns.

- fix column names after read_gtf so they work with GenomicFeatures
- add flag chain, make False by default to to_* methods
- genomicfeatures: add tss/tes-methods
- fix column names after read_gtf so they work with GenomicFeatures
- remove Strand column with unstrand() even if PyRanges is not stranded
- reading gff and gtf now consistent and column names from attributes are in lower_snake_case