fix(ifort): address issue with Intel OneAPI ifort compilation

[jdhughes-usgs]

Modified fastTwoDiff and fastTwoSum in order to compile with OneAPI ifort (2021.5.0 20211109). Added an interface to m_geometry for fastTwoDiff and fastTwoSum. Prior to modification got the following error

/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(230): error #6115: A separate interface body must have been declared in the program unit or an ancestor of the program unit for the separate module procedure.   [FASTTWODIFF]
  module subroutine fastTwoDiff(a, b, x, y)
--------------------^
/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(234): error #6451: A dummy argument name is required in this context.   [A]
    real(r64), intent(in) :: a
-----------------------------^
/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(236): error #6451: A dummy argument name is required in this context.   [B]
    real(r64), intent(in) :: b
-----------------------------^
/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(238): error #6451: A dummy argument name is required in this context.   [X]
    real(r64), intent(out) :: x
------------------------------^
/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(240): error #6451: A dummy argument name is required in this context.   [Y]
    real(r64), intent(out) :: y
------------------------------^
/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(249): error #6115: A separate interface body must have been declared in the program unit or an ancestor of the program unit for the separate module procedure.   [FASTTWOSUM]
  module subroutine fastTwoSum(a, b, x, y)
--------------------^
/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(253): error #6451: A dummy argument name is required in this context.   [A]
    real(r64), intent(in) :: a
-----------------------------^
/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(255): error #6451: A dummy argument name is required in this context.   [B]
    real(r64), intent(in) :: b
-----------------------------^
/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(257): error #6451: A dummy argument name is required in this context.   [X]
    real(r64), intent(out) :: x
------------------------------^
/Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90(259): error #6451: A dummy argument name is required in this context.   [Y]
    real(r64), intent(out) :: y
------------------------------^
compilation aborted for /Users/jdhughes/Documents/Development/coretran/src/spatial/sm_geometry.f90 (code 1)
make[2]: *** [CMakeFiles/coretran.dir/spatial/sm_geometry.f90.o] Error 1
make[1]: *** [CMakeFiles/coretran.dir/all] Error 2
make: *** [all] Error 2

closes #28 reopen issue if sm_maths_d1D.f90 issue still exists