Change pycore.rst title
Some quick bug fixes:
 * Revert a previous change that, as well as clarifying the types of vector_base
   and matrix_base objects, caused a large increase in test failures. This
   needs more investigation, as it breaks creating objects from
   pyopencl.array.Array objects whose dtype is not the default dtype;
 * Fix opencl_support for the new vector of shared_ptr<program> in context;
 * Compute norm_frobenius using matrix_base, to support slices etc.