Read the specification for a weighting type from a spec file.

Parameters

[in,out]	file	Spec file.
[out]	weighting_type	Aerosol weighting scheme.
[out]	exponent	Exponent for power-law weighting (only used if `weight_type` is `AERO_STATE_WEIGHT_POWER`).

The weighting function is specified by the parameters:

weight_type (string): the type of weighting function — must be one of: flat for flat weighting, flat_source for flat weighting by source, power for power weighting, power_source for power source weighting, nummass for number and mass weighting, and nummass_source for number and mass weighting by source. If weight_type is power or power_source then the next parameter must also be provided:
- weighting_exponent (real): the exponent for power or power_source. Setting the exponent to 0 is equivalent to no weighting, while setting the exponent negative uses more computational particles at larger diameters and setting the exponent positive uses more computaitonal partilces at smaller diameters; in practice exponents between 0 and -3 are most useful.