generate DOI on Genodo for citation purpose

`SnpData` updates and `reorder!` (#45)

* snp_info reader

* typo

* concatenation

* modified notebook

* minor modification on notebook

* OpenCL kernels from klkeys/PLINK.jl

* added a notebook for GPU

* fixed syntax for using Requires

* print platform and device info on gpu_example.jl

* add verbose to filter and modify filter in snpdata

* add reorder

* update default for filter

* modify cat functions for updated structure of snpdata

* Update reorder.jl

* fix filter for SnpData

* doc and test for reorder

* quickfixes

* change test

* simplify merge test

* remove temp data files