Cleanup interpolators and sparse functions

[navjotk]

• If LinearInterpolator._interpolate/_inject could accept sfunction as parameters, no state would be required in the LinearInterpolator class and it could be singleton-ized (see https://github.com/devitocodes/devito/pull/1025#discussion_r358091918 )
• The initialization of Interpolator objects could be homogenised across the SparseFunction hierarchy (see https://github.com/devitocodes/devito/pull/1025#discussion_r358092881 )
• Move Evaluable to devito/types/evaluable.py (see https://github.com/devitocodes/devito/pull/1025#discussion_r363208996 )