Fix ROCK methods

[deeepeshthakur]

@ChrisRackauckas

1. Fixes constants for ROCK2 and ROCK4
2. Fixes perform_step for ROCK2 and ROCK4
3. Adds min_stage and max_stage functionality in ROCK methods
4. Adds tests for min_stage. This also insures that the right constants are chosen.

[deeepeshthakur]

@ChrisRackauckas
This could a weak proof that stage selection is working fine. You see after we introduce some minimum stages, the error goes down.

1. ROCK2 1D linear
2. ROCK2 2D linear
3. ROCK4 1D linear
4. ROCK4 2D linear

[codecov]

Codecov Report

Merging #740 into master will increase coverage by 0.01%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##           master     #740      +/-   ##
==========================================
+ Coverage   71.62%   71.63%   +0.01%     
==========================================
  Files          89       89              
  Lines       29012    29022      +10     
==========================================
+ Hits        20779    20791      +12     
+ Misses       8233     8231       -2

Impacted Files	Coverage Δ
src/caches/rkc_caches.jl	96.59% <ø> (ø)	⬆️
src/tableaus/rkc_tableaus.jl	100% <ø> (ø)	⬆️
src/algorithms.jl	96.85% <100%> (+0.02%)	⬆️
src/perform_step/rkc_perform_step.jl	85.55% <100%> (+0.44%)	⬆️
src/rkc_utils.jl	78.48% <66.66%> (ø)	⬆️

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[coveralls]

Coverage decreased (-5.5%) to 69.174% when pulling f46af6c on deeepeshthakur:ROCKIMplementation into d3247e0 on JuliaDiffEq:master.