allow grids to be constructed using the Eclipse DVX, DVY, and DVZ keywords

for this purpose, a new function called 'ecl_grid_alloc_dxv_dyv_dzv'
is introduced. Also, the semantics of the 'offset' argument of the
ecl_cell_init_regular() function needed to be changed: Now it
represents the offset of the cell in question instead that of the
grid...
add simplistic unit test for ecl_alloc_grid_dxv_dyv_dzv()

so far, it just tests that the the cell volumes are correct.