Deprecation leading into v0.8

[jishnub]

1. Functions specifically applied to ProductSplit: extremadims, extrema_commonlastdim and procrange_recast -- won't be exported anymore, but are available to be imported
2. ntasks(x) is deprecated in favor of prod(length, x)
3. minimum(::ProductSplit, dim), maximum(::ProductSplit, dim) and extrema(::ProductSplit, dim) are deprecated in favor of the keyword-argument versions. Similarly nelements(::ProductSplit, dim) is deprecated in favor of nelements(::ProductSplit; dim)
4. showprogress is deprecated in pmapreduce*
5. Functions to query a cluster are are moved to a submodule ClusterQueryUtils. These won't be exported in the future.
6. gethostnames is deprecated and renamed to hostnames

[codecov]

Codecov Report

Merging #4 (d16160f) into master (e396948) will increase coverage by 0.00%.
The diff coverage is 99.75%.

@@           Coverage Diff           @@
##           master       #4   +/-   ##
=======================================
  Coverage   99.71%   99.71%           
=======================================
  Files           7        8    +1     
  Lines        1051     1061   +10     
=======================================
+ Hits         1048     1058   +10     
  Misses          3        3           

Impacted Files	Coverage Δ
src/ParallelUtilities.jl	100.00% <ø> (ø)
src/reductionfunctions.jl	96.77% <ø> (ø)
src/mapreduce.jl	99.48% <99.33%> (+0.01%)	⬆️
src/clusterquery.jl	100.00% <100.00%> (ø)
src/productsplit.jl	100.00% <100.00%> (ø)
src/utils.jl	100.00% <100.00%> (ø)

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