adding test of linking with nested rigid bodies
Load non-rigid nested rigid bodies correctly.

Relates #845.
Use IMP info if we don't have useful RMF info.

When linking to an RMF file, if we haven't read a frame yet
we can only see the static values, and for nonrigid members
that are also rigid bodies, there are no such values for
translation/rotation, so they are not read in correctly.
But IMP knows that the member is nonrigid and is a rigid
body, so use that information.

Closes #845.
Use correct class.
Fix class name.
Don't include nonrigid member rigid bodies.

This function is only supposed to return rigid
members (and rigid nested rigid bodies), not non-rigid
nested rigid bodies.
Get latest PMI.
Add 2.3.1 changes.
adding GaussianEMRestraint to isd
adding atomic xl restraint to isd
can now cast atomic xl restraint from restraint
adding function to sample points from em density
dont need module name
No need for cast(); use existing Object::get_from().
Remove unnecessary imports.
Remove unnecessary import.
Work around format checks getting confused.
Mark expensive test as such.
Fix header.
Don't export header-only functions.