C flag fix and comments

Created by: dyang37

This PR contains multiple things:

1. Changed flag of prox_mode from float to char, and modified icd update rule as proposed and discussed in issue #64 .
2. Changed flag of positivity from int to char.
3. In C code, replaced all tabs with four spaces.
4. In C code, added comments to recon() function in interface.c.

Tested by running demp_mace3D_old.py. The results of both mace3D and qGGMRF recon are visually the same as before.

Created by: cabouman

Seems like it works

Merged by: cabouman at 2022-05-20 22:40:37 UTC