Refactoring of CField struct to include gridindexingtype and interp_method

[erikvansebille]

Following #1612, where we need to provide the gridindexingtype on the CField struct for technical reasons, it makes sense to also use it throughout the rest of the C-code. Hence, this PR refactors the code to not require the gridindexingtype and interp_method when calling a Field evaluation in C, but get them from the CField itself