Add easy possibility to create array of variables

[saschatimme]

This extends the variable / parameter generation macro to allow easy creation of arrays of variables. Closes #102.

Examples:

julia> @variables x[1:3];
julia> x
3-element Array{Operation,1}:
 x[1]()
 x[2]()
 x[3]()

# support for arbitrary ranges and tensors

julia> @variables y[1:3,1:5:10,1:4];

julia> y
3×2×4 Array{Operation,3}:
[:, :, 1] =
 y[1,1,1]()  y[1,6,1]()
 y[2,1,1]()  y[2,6,1]()
 y[3,1,1]()  y[3,6,1]()

[:, :, 2] =
 y[1,1,2]()  y[1,6,2]()
 y[2,1,2]()  y[2,6,2]()
 y[3,1,2]()  y[3,6,2]()

[:, :, 3] =
 y[1,1,3]()  y[1,6,3]()
 y[2,1,3]()  y[2,6,3]()
 y[3,1,3]()  y[3,6,3]()

[:, :, 4] =
 y[1,1,4]()  y[1,6,4]()
 y[2,1,4]()  y[2,6,4]()
 y[3,1,4]()  y[3,6,4]()

# also works for dependent variables
julia> @parameters t; @variables z[1:3](t);
julia> z
3-element Array{Operation,1}:
 z[1](t())
 z[2](t())
 z[3](t())

If the design is accepted, I will add some documentation to the readme :)

ToDo:

• Documentation

[codecov]

Codecov Report

Merging #153 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   94.75%   95.01%   +0.26%     
==========================================
  Files          11       11              
  Lines         381      401      +20     
==========================================
+ Hits          361      381      +20     
  Misses         20       20

Impacted Files	Coverage Δ
src/variables.jl	94.54% <100%> (+3.11%)	⬆️

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[ChrisRackauckas]

This looks great to me. Docs?

[saschatimme]

Added the docs. Let me know if something could be improved.