Timestepper cleanup

[noelchalmers]

Some cleanup in the TimeSteppers before adding in memory pool functionality.

• Splits PML and non-PML timesteppers into distinct objects. Using inheritance here doesn't reduce much code duplication and makes the PML timesteppers harder to follow Walking this back a bit. Having the timesteppers split leads to a lot of code duplication. Instead, this unifies the pml and non-pml timestepping and using std::optional to toggle between the different run modes.
• Solvers with PMLs also should pass their PML fields into the timestepper, rather than the timestepper creating them and assuming they're zero
• Avoid initializing time stepper workspaces to zero. Instead just guard some accesses in kernels. This also reduces data motion in some kernels.
• Use async copies where possible to avoid unnecessary H<->D syncs
• Fixed a subtle bug in SARK timesteppers. Changed the bns tests to time step for longer to strengthen the test

[codecov]

Codecov Report

Merging #80 (b958088) into staging (5aa749e) will increase coverage by 0.14%.
The diff coverage is 96.61%.

@@             Coverage Diff             @@
##           staging      #80      +/-   ##
===========================================
+ Coverage    85.90%   86.04%   +0.14%     
===========================================
  Files          284      284              
  Lines        19889    19762     -127     
===========================================
- Hits         17086    17005      -81     
+ Misses        2803     2757      -46     

Impacted Files	Coverage Δ
solvers/bns/bns.hpp	100.00% <ø> (ø)
libs/timeStepper/timeStepperMRAB3.cpp	91.86% <88.46%> (+10.12%)	⬆️
libs/timeStepper/timeStepperSAAB3.cpp	95.18% <93.93%> (+1.49%)	⬆️
libs/timeStepper/timeStepperMRSAAB3.cpp	95.00% <94.73%> (+7.04%)	⬆️
libs/timeStepper/timeStepperSARK4.cpp	97.63% <96.55%> (+3.75%)	⬆️
libs/timeStepper/timeStepperSARK5.cpp	97.86% <96.77%> (+3.52%)	⬆️
include/timeStepper.hpp	66.66% <100.00%> (+3.03%)	⬆️
libs/timeStepper/timeStepper.cpp	100.00% <100.00%> (ø)
libs/timeStepper/timeStepperAB3.cpp	100.00% <100.00%> (ø)
libs/timeStepper/timeStepperDOPRI5.cpp	100.00% <100.00%> (+0.74%)	⬆️
... and 11 more

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