Ensure that input data/parameters are accessible in output files

[astrofrog]

Need to decide if this should be done by linking, or copying. Either would work, but if using linking, then users would have to ensure that input file is kept, otherwise certain outputs would not be readable.

Maybe the best idea is to always copy, but to give users the option to link if they care about performance (maybe should be a general rule that would apply to dust properties too). This would require a special page in the manual on reducing file sizes by using the optional linking?

[astrofrog]

Implemented linking in 3adb04c6330aad024f35534c4f980c6ce581035b and copy in 36efd574710fd9f8b56e970eecdedfd02e391bc1 and fea105363654b2b72f531d86b6ea86d894ae4b64. The default is to copy, but this can be changed with:

m = Model(...)
...
m.set_copy_input(False)

in which case the input is linked.

[astrofrog]

Documentation about copying/linking added in d84fcfda53c24eeb742a8d509ff13c9f97445ef9.