Options to interpolate e3U-V-F with old non-lin fs

Added new namelist variable "nn_vvl_interp" to control the interpolation scheme The default(=2) is set to a "qco" like formulation

Closes #6 (closed)

changed milestone to %Release 4.2

requested review from @acoward

assigned to @jchanut

requested review from @gsamson and removed review request for @acoward

requested review from @acoward and removed review request for @gsamson

merged

mentioned in commit 32318db1