Fixed branch cut of polylog again

This fixes the problem of 3b2d170d and bdc545ff
for good.

In the previous version I had a conjg in there,
but that turned out to be nonsense. To see why,
see the comment in 3b2d170d (spoiler alert: it has
to do with a peculiarity of IEEE 754)

src/maths_functions.f90

@@ -400,6 +400,19 @@ CONTAINS
 
   END FUNCTION
 
+  FUNCTION MYLOG(x)
+    complex(kind=prec) :: x, mylog
+    if (abs(aimag(x)) < zero) then
+      if (real(x) > 0) then
+        mylog = cmplx(log(real(+x,kind=prec)),     kind=prec)
+      else
+        mylog = cmplx(log(real(-x,kind=prec)), pi, kind=prec)
+      endif
+    else
+      mylog = log(x)
+    endif
+  END FUNCTION
+
   RECURSIVE FUNCTION polylog1(m,x) result(res)
     ! computes the polylog
     
@@ -433,7 +446,7 @@ CONTAINS
     else if (abs(x) .gt. 1) then
       inv = 1._prec/x
       res = (-1)**(m-1)*polylog(m,inv) &
-          - (2._prec*pi*i_)**m * bernoulli_polynomial(m, 0.5_prec-i_*log(-x)/2._prec/pi) / factorial(m)
+          - (2._prec*pi*i_)**m * bernoulli_polynomial(m, 0.5_prec-i_*mylog(-x)/2._prec/pi) / factorial(m)
     else if(m == 2) then
       res = dilog(x)
     else if(m == 3) then
@@ -489,7 +502,7 @@ CONTAINS
       plog1 = plog1 - b%i0*i_*pi
     endif
   else
-    plog1 = log(1.-a%c/b%c)
+    plog1 = mylog(1.-a%c/b%c)
   endif
   END FUNCTION