change parameters to radians from degrees in oslo_aero_hetfrz

[mvertens]

The following in degrees - but is only used in radians.

real(r8),parameter :: theta_imm_bc = 48.0_r8 ! contact angle [deg], converted to rad later !DeMott et al (1990)
real(r8),parameter :: deg2rad = pi / 180.0_r8
real(r8),parameter :: theta_imm_bc = 48.0_r8 * deg2rad ! contact angle [rad] !DeMott et al (1990)