Reformulated computation of k

This is for #87
Changed initial algorithm to initial equation

This is for #87
Added parameter keyword.

This is for #87 and needed for OpenModelica.
Removed the end keyword

This is for #87 and needed for OpenModelica.
Changed value of min attribute of dpValve_nominal

This is for #87.
Merge pull request #88 from iea-annex60/issue87_valves

Issue87 valves