Some initial files added to support RigidIBFE

A few comments:

(1) Both the regular IBStrategy and the rigid IBStrategy should use the same spreadForce() and interpolateVelocity() functions. The rigid IBStrategy should not replicate this functionality. One possibility is to split IBStrategy into three parts:

   IBStrategy - basic functions that know how to spread, interpolate, etc but not to compute forces
   IBElasticStrategy - functions specific to computing elastic forces
   IBRigidStrategy - functions specific to rigid-body motion

(2) The rigid IBFEMethod should either simply be IBFEMethod, or it should be a subclass of IBFEMethod.

(3) Class names should start with uppercase. Prefer the convention of names becoming more specific as you read them from left to right (e.g. IBRigidStrategy rather than RigidIBStrategy).