Add remaining scalar-output functions from Anna Lea's code

[leifdenby]

From https://github.com/annalea-albright/EUREC4A/blob/master/eurec4a_hackathon/describe_vertical_structure_HALO_circles.py there are a number of functions still to refactor:

• find_ml_height_from_gradient -> z_dthetav_dz, z_dq_dz_0
• calc_peak_RH_linearize -> z_BLsat_linfit, issue Determining mixed-layer depth from linfit of mixed-layer values #13
• calc_peak_RH -> z_maxRH, issue Determining mixed-layer depth from relative humidity peak #12
• calc_density
• calc_static_stability -> static_stab_profile
• find_inversion_height_static_stability -> z_inv_static_stability
• find_inversion_height_grad_RH -> z_inv_gradRH
• find_inversion_height_grad_RH_and_T -> z_inv_gradRH_T