Support per voxel mua/mus/g/n continuous medium type

[fangq]

MCX already supports setting per-voxel mua/mus values, but it does not support varying mua/mus/g/n all together for each voxel.

This feature has been requested a few times in the forum

https://groups.google.com/g/mcx-users/c/4wnSBCjaRH8/m/D_eGMqhZBwAJ
https://groups.google.com/g/mcx-users/c/OleiBgSzG3E/m/qagEagp0BAAJ
https://groups.google.com/g/mcx-users/c/2gC8iqydg48/m/glBB8vqTAAAJ

so, supporting this feature seems to be beneficial although users should be warned that one shall not abuse this feature - using label-based simulation still offers the highest computational efficiency over using per-voxel medium format.