MAINT: symeig -- now that's a name I've not heard in a long time

The lobpcg code has some references to `symeig` which has been replaced by `eigh` in scipy.  This PR removes those old references.

The code also has some non-working benchmarks and tests which I've partially updated, but I've run into some difficulties so this part is not yet finished.  One difficulty is that `lobpcg` is the name of a subpackage, a module, and a function, and this is causing some import weirdness.  Anyone have a suggestion as to how these files or functions should be renamed?

Another difficulty is that lobpcg uses numpy arrays, matrices, and abstract linear operators, and the old non-working testing and benchmarking code seems to attempt to use an obsolete interface to lobpcg.  I might try to fix this in the future.  For example this old non-working code attempts to pass a plain python function that linearly maps vectors to vectors, whereas the current lobpcg interface expects something more like a LinearOperator or array or matrix to represent such a linear operator.  The state of generic linear operator interfaces in numpy/scipy is somewhat dismal.  For example here's [yet another](https://github.com/SteveDiamond/AbstractLinear) new github project added a couple weeks ago to try implementing such an interface.