Testing mat operations with vec

[sebsjames]

To make sure mat += vec does NOT work, and similar. See #113 for the query that motivated this.

Previously

mat += or -= vec worked (even with vec3/vec4 for mat<F, 4>)
mat + or - vec worked
mat == vec worked
mat != vec worked
mat = vec worked (assignment)
you could construct mat<F, 4> from a vec<F, 3> or vec<F, 4> (creating a translation matrix)

Now

mat = vec works if vec size is Nr * Nc (only)
mat == vec doesn't compile
mat != vec doesn't compile
mat +=/-=/+/- vec don't compile
you can't construct mat<F, 4> from a vec<F, 3> or vec<F, 4> (too confusing)
you can still construct mat from a vec of the right size, assumed to contain data in col major order

[sebsjames]

Oh, I should add: Big thanks to @iboB for the test-must-fail-to-compile code that I have used here!