[grids] use float_cmp instead of np.isclose in flatten_grid
[operators.fv] use diag_matrix instead of diags for compatibility with older SciPy versions
[tools] remove unnecessary future imports in pyx files

to be compatible with older Cython versions
[demos] remove obsolete 'reductor' parameter in thermalblock demo
[operators.basic] re-implement assemly of NumpyLincombMatrixOperator

in particlar to support older SciPy versions