Remove dynamic dispatching from field computation code.

[toma-vlad]

The code pattern

f(args..., symbol) = f(args..., Val(symbol))

should be removed from the code (1, 2 and 3) as it greatly affects performance. Or at least we should change definitions as follows

f(args...) = f(args..., Val(:real))

to maintain performance where it counts.

[SebastianM-C]

#53 should have fixed this.