Revisit GSL error handling.

[molpopgen]

#101 introduced an "init" module that promotes GSL errors to C++ exceptions. This approach works well when fwdpy11 is the only module using GSL. However, this behavior conflicts with msprime, which disables all GSL error handling (see tskit-dev/msprime#632).

A preferable behavior may be scoped unsetting of the global handler, resetting upon exit. We already have the classes in place to do this.

[molpopgen]

py::call_guard may be the way to go here. Instead of trying to handle things globally upon import, we can do it on a case-by-case-basis.