v5.1.0

Aims for version 5.1:

GlobalLinSys restructuring of local coefficients
MultiRegions class hierarchy restructuring to remove dimension-dependent classes
Initial version of the implicit CompressibleFlowSolver
Time integration scheme refactoring & defect correction scheme
CardiacEPSolver improvements: tests and tutorials
Transition development model to use git forks vs. central repository
Improvements to tutorials: new basics tutorials, Jupyter tutorials more mainstream, tutorials from examples that were removed in v5 & further improvements from developments for workshop in Shanghai