Pyranges

- fix merge print functions
- make pickleable
- add iter as alias for itergrs in pr. namespace
- gr.length() shows nucleotide length (sum of all interval lengths)
- gr.lengths() takes as_dict=False flag to return as vector
- fix slack in join: added columns when joining with itself
- fix print for unstranded pyranges: printed tail and head of first chromosome

- add overlap-flag to tile
- add chain to print-method
- bugfix: printing stranded pyranges sorted output even though sort was false
- bugfix: wrong number hidden cols on very small terminal widths
- bugfix: unstrand did not change underlying dict to chromosomes only
- show number of hidden columns in header
- tests: mismatches in strand between dict and dataframes
- .df/.as_df() now returns with non-duplicated index

- add possibility of 5-end and 3-end in slack being different/none
- add slack to join-method
- add new_position method to take union or intersection of two pairs of Start/End-columns in pyranges

- Add int64-flag to method pr.random.

- Ensure that Chromosome and Strand is str dtype before creating category
- Add check to ensure that the columns Chromosome, Start and End exist when trying to create a PyRanges

- Fix error in pypi file