This patch defines a helper function, _xlog2x(x), that calculates
x * log2(x) but handles the case x == 0 by returning 0 rather than nan.
This is needed if the power spectrum has any component that is exactly
zero: in particular, if the f = 0 component is zero.

• Commit 6192233
• #3: Handle the limit of p = 0 in p log2 p
• Branch master

jftsang