fix instantiation issue at link time
fix #348
 - implement evaluation over faces
 - added `minmax` function to compute the minimum and maximum of an expression
 - work in parallel
 - allow passing your own worldcomm to `normLinf` and `minmax`
update  #348
 - add interface to normLinf and minmax results
 - use this interface in test_evaluator
update #347
 - use minmax (see #348)
 - rename local  reynolds number to Peclet number
 - min and max of uz and the normalized value (there seem to be an issue here or in
minmax)