Add DynamicalSDEFunction and Problem

[jamesgardner1421]

I've added the DynamicalSDEProblem and DynamicalSDEFunction. I'm not entirely sure if these are necessary since they're almost direct copies of the existing SplitSDEProblem/Function.

The reason for this PR is for #259 in StochasticDiffEq. I've also submitted a PR to StochasticDiffEq that uses these functions with the BAOAB algorithm.

I don't understand the existence of the SplitSDEProblem type since its constructors just create the original SDEProblem instead, so as far as I can tell, the SplitSDEProblem itself is never used. I followed this trend for the new DynamicalSDEProblem but perhaps it's not necessary.

[codecov]

Codecov Report

Merging #21 (4d17fce) into master (dbcf70f) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master     #21      +/-   ##
=========================================
- Coverage    2.87%   2.85%   -0.03%     
=========================================
  Files          37      37              
  Lines        2712    2734      +22     
=========================================
  Hits           78      78              
- Misses       2634    2656      +22     

Impacted Files	Coverage Δ
src/SciMLBase.jl	50.00% <ø> (ø)
src/problems/sde_problems.jl	0.00% <0.00%> (ø)
src/scimlfunctions.jl	2.88% <0.00%> (-0.05%)	⬇️
src/solutions/solution_interface.jl	0.00% <0.00%> (ø)

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