cast boolean to int in evaluable.asindex
cast boolean to int in evaluable.asindex

The `asindex` function casts its input to an array and asserts that the
dimension is zero and the dtype as `bool` or `int`. Callers of this function
expect the return value to have dtype `int` unconditionally. This patch casts
the value to an `int` if necessary.

wijnandhoitinga/nutils