python interface for DirichletBC.markers() missing in 2018.1.0

[Aslak Bergersen]

In version 2018.1.0, DirichletBC is missing the attribute markers() in the Python interface. However, markers is listed in C++ DirichletBC class cell.

Of note is also that user_sub_domain is renamed in the Python interface to user_subdomain.

I'm assuming this issue is similar to #1037