Revision of the STRICT_MODE concept

[ppravatto]

Modify the strictness control now defined by STRICT_MODE according to three levels of strictness:

1. Default: allow different levels of theory for different properties, provided no conflicts arise, otherwise refuse to mix.
2. STRICT: requires equality across electronic and vibronic levels of theory.
3. VERY_STRICT: requires equality across geometry, electronic, and vibronic levels of theory.