v0.4.0

Top-line updates -- note that this version of pyqmc will not give results that are compatible with version 0.3.0.

• Reorganization of the API; check the documentation; you will probably want to import pyqmc.api instead of just pyqmc
• DMC algorithm is now using Umrigar's new algorithm with very small time step errors.
• Redesign of Slater and multiple Slater wave function objects. Now support multiple Slater determinants for periodic systems
• Initial GPU implementation using cupy. We have

Minor changes

• Superposition wave function
• Updated documentation
• DMC now support complex wave functions
• Performance improvements.
• Various bugfixes, in particular for PBCs.