Support simple arithmetic expressions in computed fields

[johnstairs]

Adding the following to computed field expressions:

1. Floating-point literals (1.2)
2. Simple arithmetic ((2 + 3) ** 4)
3. Type conversions (myfield as int)

In a future change, we add more operators like | and << and support such expressions in enum and flag value definitions.

Addresses #73

[hansenms]

This looks great. We can do more expansion of this later, e.g. adding boolean fields....isSquare(size(arr,0) == size(arr,1)), etc. But let us let that evolve as the need arises.