Add underlying array type to enable GPU use.

[aterenin]

This PR adds the type of the underlying vector to the ElasticArray struct. This enables one to use a non-Vector type, for instance a CuArray, with the package. It also adds a constructor so that Flux.@functor works with ElasticArrays out-of-the-box. The arrays appear to work on GPU just fine, albeit with scalar getindex, which is slow but nonetheless functional.

[codecov-io]

Codecov Report

Merging #16 into master will decrease coverage by 1.78%. The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage     100%   98.21%   -1.79%     
==========================================
  Files           3        3              
  Lines          54       56       +2     
==========================================
+ Hits           54       55       +1     
- Misses          0        1       +1

Impacted Files	Coverage Δ
src/elasticarray.jl	98.11% <80%> (-1.89%)	:arrow_down:

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[oschulz]

Nice, thanks!

[oschulz]

Will make a new release with this, but need to migrate CI from travis-ci.org to travis-ci.com first (in progress).