Improve namespace qualification of min/max, other minor code improvements

• Primary purpose is to avoid compile conflict with Foam::min(), Foam::max() with unqualified versions that may be included with AMD or nvidia compiler headers.
• Provide some efficiency improvements when clamping field min/max values, as well as reciprocal time values.
• reduce the number of places where an implicit cast from UList to List is required, since this can open the possibility of adding more member data into List in the future without encountering casting issues.