Control functions in Z alpha

[giacrossi]

The functions weno_exp and tau maybe have to be changed in something new according to the following papers:

High Order Weighted Essentially Non-Oscillatory WENO-Z schemes for Hyperbolic Conservation Laws, Castro, Marcos and Costa, Bruno and Don, Wai Sun, JCP, 2011, vol. 230, pp. 1766--1792, doi:10.1016/j.jcp.2010.11.028

Accuracy of the weighted essentially non-oscillatory conservative finite difference schemes, Don, Wai-Sun and Borges, Rafael, JCP, 2013, vol. 250, pp. 347--372, doi:10.1016/j.jcp.2013.05.018

An improved weighted essentially non-oscillatory scheme for hyperbolic conservation laws, Borges, Rafael and Carmona, Monique and Costa, Bruno and Don, Wai Sun, JCP, 2008, vol. 227, pp. 3191--3211. doi:10.1016/j.jcp.2007.11.038

[giacrossi]

The weno_exp function has been deleted in commit 8b0c41b0c55127653d2d78bd6d82a59f76eee275, but two issues remain open:

1. the role of the exponent in the tau function;
2. the expression used to evaluate tau function.