Operator constructors silently accept zero, negative, NaN and Inf for dx/dy/dz

[gpagallo]

Describe the bug
The values dx/dy/dz are currently accepted unchecked. Passing dx = 0 produces
silent Inf/NaN propagation through the assembled operator (the 1-D
Gradient and Divergence constructors both end with *this /= dx;); passing
a negative spacing yields a sign-flipped operator with no warning.

To Reproduce
There is no input validation for the cell-spacing arguments (dx, dy, dz)
across every C++ operator constructor and AddScalarBC free function.

Expected behavior
Each spacing will now be required to be strictly positive and finite;
violations throw std::invalid_argument with a descriptive message naming
the offending parameter and value.

Screenshots
NA

Desktop (please complete the following information):

• OS: MacOS 26.5
• Browser Safari
• Version: 26.5

Smartphone (please complete the following information):
NA

Additional context
NA

Keep-open request

• I am requesting maintainer review for keep-open.

Reason:
Make MOLE C++ more resilient to input errors