Change warning in lambdify to be less obnoxious

Now:
- only warns if `expr.is_Matrix`
- only warns once

Fixes #8030.
Rewording suggestion:

Currently, sympy.Matrix is replaced with numpy.matrix if the NumPy package is utilized in lambdify. In future versions of SymPy, we will default to replacing sympy.Matrix with numpy.array. To use the future behavior now, supply the kwarg 'default_array=True'. This kwarg will be removed in 0.7.7 but the old behavior can be retained in future versions by passing in a custom dictionary to the 'modules' kwarg.