Radial_mesh stuff and evaluation of selected equation(s) 

Hi,
I send a pretty big pull request. I suspose, that you don't wann merge whole branch, so I mention here the commits, that would be cool to have it in main repository, I suspose (I partially tried it), that they can be cherry picked without (bigger) problems. 

Evaluation of selected (via conf created) equation via
problem.eval_equations('eq_name', ....) 
problem.eval_equations(['eq1_name', 'eq2_name', ...], ....) 
a4084e11301de0dc54a5c73cf8c10a96cce6efc2

Radial vector/mesh and potentials stuff (directory physics)
a50f74f534c6527e779918f50c3752876960c3e8..ab17448e8c811293a7cdea14267e00210a399543
2f732763b1bbcc0b9de9b3deafc9f9837698dff6
e176f061469d02d1f3cc14bc611f3ed206068d18..8128bf1b69660b2b870f2019f21416555e52c71a
0d208a897f4a7cc0f414f29758605d792c976f8e

Parsing commandline options (minor changes)
56c01c9e57664f6b3536c6c4bff79957d857fbc7
b2fc8a39691ca029bb6e26bde7321311b5b7719e