Move out DiffEqScaledOperator

[SebastianM-C]

This moves out the following to SciMLBase

• DiffEqScaledOperator and its methods
• AbstractDiffEqCompositeOperator
• getops

The SciMLBase package is also added as a dependency.
The compat entry for SciMLBase should be added after the required version is registered.

This should be tested together with SciML/SciMLBase.jl#44

[SebastianM-C]

Running the tests locally together with the SciML/SciMLBase.jl#44 PR gives the following test error:

Validate Higher Dimensional Boundary Extension: Error During Test at C:\Users\sebastian\.julia\packages\SafeTestsets\A83XK\src\SafeTestsets.jl:25
  Got exception outside of a @test
  LoadError: BoundsError: attempt to access 1-element Base.OneTo{Int64} at index [2]
  Stacktrace:
    [1] throw_boundserror(A::Base.OneTo{Int64}, I::Int64)
      @ Base .\abstractarray.jl:651
    [2] getindex
      @ .\range.jl:702 [inlined]
    [3] _broadcast_getindex_evalf
      @ .\broadcast.jl:648 [inlined]
    [4] _broadcast_getindex
      @ .\broadcast.jl:621 [inlined]
    [5] #19
      @ .\broadcast.jl:1098 [inlined]
    [6] ntuple
      @ .\ntuple.jl:50 [inlined]
    [7] copy
      @ .\broadcast.jl:1098 [inlined]
    [8] materialize
      @ .\broadcast.jl:883 [inlined]
    [9] getindex
      @ .\multidimensional.jl:353 [inlined]
   [10] _getindex
      @ .\abstractarray.jl:1214 [inlined]
   [11] getindex(A::CartesianIndices{3, Tuple{Base.OneTo{Int64}, UnitRange{Int64}, UnitRange{Int64}}}, I::CartesianIndex{3})
      @ Base .\abstractarray.jl:1170
   [12] Array(Q::DiffEqOperators.ComposedMultiDimBC{Float64, DiffEqOperators.AtomicBC{Float64}, 3, 2}, s::Tuple{Int64, Int64, Int64})
      @ DiffEqOperators ~\.julia\dev\DiffEqOperators\src\derivative_operators\concretization.jl:242
   [13] top-level scope
      @ ~\.julia\dev\DiffEqOperators\test\multi_dim_bc_test.jl:65
   [14] include(mod::Module, _path::String)
      @ Base .\Base.jl:386
   [15] include(x::String)
      @ Main.##269 ~\.julia\packages\SafeTestsets\A83XK\src\SafeTestsets.jl:23
   [16] macro expansion
      @ ~\.julia\dev\DiffEqOperators\test\runtests.jl:23 [inlined]
   [17] macro expansion
      @ C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Test\src\Test.jl:1151 [inlined]
   [18] top-level scope
      @ ~\.julia\dev\DiffEqOperators\test\runtests.jl:23
   [19] eval(m::Module, e::Any)
      @ Core .\boot.jl:360
   [20] macro expansion
      @ ~\.julia\packages\SafeTestsets\A83XK\src\SafeTestsets.jl:23 [inlined]
   [21] macro expansion
      @ timing.jl:210 [inlined]
   [22] macro expansion
      @ ~\.julia\dev\DiffEqOperators\test\runtests.jl:23 [inlined]
   [23] top-level scope
      @ timing.jl:210
   [24] include(fname::String)
      @ Base.MainInclude .\client.jl:444
   [25] top-level scope
      @ none:6
   [26] eval
      @ .\boot.jl:360 [inlined]
   [27] exec_options(opts::Base.JLOptions)
      @ Base .\client.jl:261
   [28] _start()
      @ Base .\client.jl:485
  in expression starting at C:\Users\sebastian\.julia\dev\DiffEqOperators\test\multi_dim_bc_test.jl:65
Test Summary:                                  | Pass  Error  Total
Validate Higher Dimensional Boundary Extension |   25      1     26
ERROR: LoadError: Some tests did not pass: 25 passed, 0 failed, 1 errored, 0 broken.
in expression starting at C:\Users\sebastian\.julia\dev\DiffEqOperators\test\runtests.jl:9
ERROR: Package DiffEqOperators errored during testing

[ChrisRackauckas]

That's just a v1.6-induced error. Mentioned in #359

[ChrisRackauckas]

Rebased.