Add missing size method for PCGALowRankMatrix

[haampie]

IterativeSolvers.jl now uses max(size(A)) in least-square solvers to determine the maximum number of iterations. This method is missing for the PCGALowRankMatrix of GeostatInversion.jl

[coveralls]

Coverage increased (+0.09%) to 85.075% when pulling 9ac00e6 on haampie:fix-size-method into d4957ac on madsjulia:master.

[codecov]

Codecov Report

Merging #2 into master will decrease coverage by 0.5%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
- Coverage   84.98%   84.47%   -0.51%     
==========================================
  Files           8        8              
  Lines         333      335       +2     
==========================================
  Hits          283      283              
- Misses         50       52       +2

Impacted Files	Coverage Δ
src/lowrank.jl	84.9% <0%> (-3.33%)	⬇️

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