write VMEC input CURTOR (#1019)

`_Profile.__call__` returns an ndarray, but we need a float to print it
as a string.

desc/vmec.py

@@ -1472,7 +1472,7 @@ def write_vmec_input(cls, eq, fname, header="", **kwargs):  # noqa: C901
         else:
             current = eq.current
             f.write("  NCURR = 1\n")  # current profile specified
-            f.write("  CURTOR = {:+14.8E}\n".format(float(current(1))))  # AC scale
+            f.write("  CURTOR = {:+14.8E}\n".format(float(current(1)[0])))  # AC scale
             if isinstance(current, PowerSeriesProfile) and current.sym:
                 f.write("  AC =")  # current power series coefficients
                 for ac in current.params: