Lifetime statistics into gmx-select.

This provides a lot more flexible alternative to g_dist -lt.

Added an analysisdata module that computes lifetime statistics and test
for it.  Currently tuned for this one application, but should be
possible to make more flexible if required.  Using this module, the
implementation in gmx-select is straightforward.

Part of #665.

Change-Id: I8bff89599c9d8a820c761b426c01ceafa9afcc5a