Adding constexpr keywords for constructors and some members.

[dbiswas2808]

More coord constructors can be constexpr. Have found this useful for writing complex finite difference operations using neighboring cells.
A simple example of usage:

Coord p; // Some input coord

// Find this pattern much easier to debug and follow for even larger number of neighbors.
constexpr Coord n0Off{1, 0};
constexpr Coord n1Off{-1, 0};
constexpr Coord n2Off{0, 1};
constexpr Coord n3Off{0, -1};

auto dx  = ((p + n0Off) - (p + n1Off)) / 2 ;
auto dy  = ((p + n2Off) - (p + n3Off)) / 2 ;

[linux-foundation-easycla]

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[dbiswas2808]

/easycla

[dbiswas2808]

Not sure why easyCLA is not getting retriggered.