Set fast AMG solver to default for upscaling applications

Change default value of linsolver_type to 3 (fast AMG) and
linsolver_prolongate_factor to 1.0.
Merge branch 'master' of https://github.com/OPM/opm-upscaling
Update documentation of options for linsolver_type
Set fast AMG solver to default in UpscalerBase

Change default value of linsolver_type to 3 (fast AMG) and
linsolver_prolongate_factor to 1.0.
Set default number of smoothing steps for linear solver to 1

The default solver fastAMG use only one smoothing step anyway
Merge pull request #68 from laods/use_fastAMG

Use fastAMG solver as default linear solver for upscaling applications