Read the specification for a kernel type from a spec file and generate it.

Parameters

[in,out]	file	Spec file.
[out]	coag_kernel_type	Kernel type.

The coagulation kernel is specified by the parameter:

coag_kernel (string): the type of coagulation kernel — must be one of: sedi for the gravitational sedimentation kernel; additive for the additive kernel; constant for the constant kernel; brown for the Brownian kernel, or zero for no coagulation