Support for 'o' and 'on' boundary conditions added to non-stress formulation.

[mattiasbr]

Enables the use of inhomogeneous 'O' and 'ON' boundary conditions (i.e. 'o' and 'on') of the form
nu dv/dy - p = pa
nu du/dx - p = pa
for non-stress formulation, where pa may be specified in userbc().

[pschlatt1]

I think this pull request is ok, and should be merged.

[fischer1]

@ananiast Does this work also for pn/pn ? Or do we need to make an error check
if this BC is used for pn/pn ?

@mattiasbr Do we have an example by which we can test this feature ? - Paul

[ananiast]

It won't be enough for Pn-Pn because we need to modify some other routines as well,
but it should be OK for Pn-Pn-2. I suggest we accept it but you should add an error check
about this

[stgeke]

@ananiast should we merge this pull request?

[stgeke]

@mattiasbr Please resolve the conflicts. Thanks!

[mattiasbr]

The conflicts are now resolved and an error check for PnPn has been added to bcmask().

[pschlatt1]

Can we merge it?

[stgeke]

@ananiast Did you implement this change with another pull request?
In this case we can close the request w/o merging.