Piecewise root finding algorithm for Euclidean projection on elastic net

1 | ```
EuclideanProjectionENNORM(y, t, alpha = 0.5)
``` |

`y` |
constant vector |

`t` |
radius of elastic net ball |

`alpha` |
parameter in elastic net: alpha x_1 + (1-alpha)*x_2^2=t |

a list containing network adjacency matrix, node score and module membership

Dong Li, dxl466@cs.bham.ac.uk

Gong, Pinghua, Kun Gai, and Changshui Zhang. "Efficient euclidean projections via piecewise root finding and its application in gradient projection." Neurocomputing 74.17 (2011): 2754-2766.

1 2 3 | ```
y=rnorm(100)
x=EuclideanProjectionENNORM(y,1,0.5)
sparistyx = sum(x==0)/100
``` |

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