gcc will now accept negative values

Fix Bug with rank function

See merge request tpurcell/cpp_sisso!62

Check magnitude with abs() not just the number

Remove warnings in the source code

See merge request tpurcell/cpp_sisso!60

provide GPU implementation for linear l0 regularization

See merge request tpurcell/cpp_sisso!51

use check_model.py not ../check_model.py

Since the python install is not working use np.allclose against
ref data