Update install guide
Small edits to mimic.rst.
Additional comments/questions:
- In the MCL compilation with tests, we need to specify with env. var. GTEST_ROOT where googletest is (if not in a system dir)
- CPMD: Probably it is better to specify that we need to export both the MiMiC and the MCL paths in case one wants to compile things in different folders.
Edited by Davide Mandelli