Pulp interface xpress py

[djunglas]

This pull request adds a new solver XPRESS_PY. Unlike the existing XPRESS solver, the new solver does not invoke an external process (the Xpress command line tool) but instead uses the Xpress Python API directly.

This is more flexible and gives access to more features (in fact all) of the Xpress optimizer.

We also introduce an alias XPRESS_CMD for the existing XPRESS solver to meet the naming conventions that were established for other solvers (CMD goes through an external command, PY uses the solver's Python API).