Add symmetric Gauss Seidel smoothing, improve smoother

[ranjanan]

Improvements to smoothing will aid convergence in most cases. This will be the default from now on.

[coveralls]

Coverage decreased (-0.1%) to 95.105% when pulling b636e5f73aa74dd4efd6cae8ca0910f7c7c8bcbf on moreopt into bdaaa7c8f4c7dc51594d6170e96e40bf69d4e2a1 on master.

[codecov-io]

Codecov Report

Merging #13 into master will decrease coverage by 0.04%. The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
- Coverage   95.23%   95.18%   -0.05%     
==========================================
  Files           7        7              
  Lines         273      291      +18     
==========================================
+ Hits          260      277      +17     
- Misses         13       14       +1

Impacted Files	Coverage Δ
src/smoother.jl	100% <100%> (ø)	:arrow_up:
src/multilevel.jl	75% <100%> (ø)	:arrow_up:
src/classical.jl	100% <100%> (ø)	:arrow_up:
src/preconditioner.jl	66.66% <50%> (-16.67%)	:arrow_down:

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[coveralls]

Coverage decreased (-0.05%) to 95.189% when pulling 5dde5547d38501fdbf44611c1b60fe977553b0f1 on moreopt into bdaaa7c8f4c7dc51594d6170e96e40bf69d4e2a1 on master.