print subproblems and/or master

[giommazz]

Would it be possible to have a function to to print pricing subproblems and/or the master?
It would be useful when debugging

[guimarqu]

@artalvpes Moving the content of the book branch in the column generation printer will close this issue.

[rrsadykov]

When printing Reformulation with :user_only flag, the master and pricing subproblem are printed with only user defined variables and constraints (artificial variables and columns are not shown):

print(IOContext(stdout, :user_only => true), reform)

Later, another flag may be defined to show master columns (together with corresponding subproblem soutions) instead of representative variables.