Fixes RK4 and AB3 timesteppers for sol<:AbstractArray

[navidcy]

• RK4substeps! function for sol::AbstractArray was missing args t and dt
• overloads AB3 timestepper so that it works with sol::AbstractArray

[navidcy]

After this is merged I'll implement what issue #100 discusses (i.e., remove Filtered versions of timesteppers).

[codecov]

Codecov Report

Merging #106 into master will decrease coverage by 0.95%.
The diff coverage is 61.11%.

@@           Coverage Diff            @@
##           master   #106      +/-   ##
========================================
- Coverage   72.95%    72%   -0.96%     
========================================
  Files           8      8              
  Lines         488    500      +12     
========================================
+ Hits          356    360       +4     
- Misses        132    140       +8

Impacted Files	Coverage Δ
src/timesteppers.jl	73.36% <61.11%> (-2.38%)	⬇️

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[coveralls]

Coverage decreased (-1.0%) to 71.888% when pulling 7236d3d on FixAbstractSolTimeStepping into 9e5817e on master.

[coveralls]

Coverage decreased (-1.0%) to 71.888% when pulling 7236d3d on FixAbstractSolTimeStepping into 9e5817e on master.