add subroutine 'slp_center'
add subroutine 'p2p_q2rh'
move subroutine 'extrapolate_Ts', 'extrapolate_Ts' to calcmet_module.f90
rename some function in calcmet_module.f90
Revise the interpolation procedures

Conversion of Ps to SLP and SLP to Ps is calculated on the gaussian
grid using 'not interpolated' model orography. This revision avoids
noise near the steep mountains in the early stages of time integrations.