Fixed least-squares for square matrices.

Used to throw a condition (which was actually undefined, fixed that
too). Arguably it could fall back to LLA:SOLVE, but this does the same
thing so it should be OK. Added test case. Thanks to Scott Turner
for reporting the bug.
Workaround for https://github.com/tpapp/lla/issues/11.
Fixed symbol clash.

LLA and CL-SLICE both had INVERT.
Merge branch 'master' of github.com:tpapp/lla