Implement correct behaviour for signed vector power operations in vvp.

Signed vector power operations were being implemented using the double
pow() function. This gave inaccurate results when the operands or
result were not exactly representable by a 64-bit floating point number.
Fix for br946 - procedural continuous assignment causes vvp internal error.

In the case that the RHS of a procedural continuous assignment is a simple
vector that is wider than the LHS, changes to the RHS vector cause the
entire vector to be sent to port 1 of the LHS vvp_fun_signal object. This
vector needs to be coerced to the size of the LHS. Note that this is a
stopgap fix until vvp handles arbitrary expressions on the RHS of a
procedural continuous assignment.
Output sorry message for missing procedural assign/force functionality.

A procedural continuous assignment is supposed to be updated any time
a variable on the RHS changes. Currently this only happens if the RHS
is a simple signal.