Add option to specify cross section area for 2D-GRM

[schmoelder]

This PR adds the option to specify the cross section area for the 2D-GRM. If COL_RADIUS is given, cross section area will be ignored.

Fixes #270

Open question: Do we need to add the parameter to the "map"?

e.g.

parameters[makeParamId(hashString("COL_LENGTH"), unitOpIdx, CompIndep, ParTypeIndep, BoundStateIndep, ReactionIndep, SectionIndep)] = &_colLength;

[jbreue16]

we dont need to add the cross section to the parameters map, as we only use it to compute the radius.

[schmoelder]

Thanks @jbreue16

we dont need to add the cross section to the parameters map, as we only use it to compute the radius.

Just out of curiosity, what does this parameters map do?

[jbreue16]

Just out of curiosity, what does this parameters map do?

You can use it to access or set parameters from outside the unit operation via a unique parameter ID