Aerosolpy

`aerosolpy.growth.VbsModel` includes particle phase diffusion.
no backward-compatibility to previous versions due to changes in method
definitions

includes subpackage `aerosolpy.growth` with classes for kinetically-limited
growth, H2SO4 growth, volatility calculation and VBS growth module

includes dayofyear_to_datetime function in time.py module

fixed bug in DMA class penetration loss calculation

includes subpackage `aerosolpy.instr` with classes for DMAs, CPC, and MPSS.

fixed an essential calculation error in
`aerosolpy.AerosolMechanics._slipcorr_deriv` which was there since the
beginning.

some fixes, and updates on documentation

first stable version. only included aerosolpy