add support for using dict inside config file

[keiyamamo]

This is draft and under testing.

[keiyamamo]

This PR adds support for using dict inside the config file. Even though we have two default variables solid/fluid_properties
that are expected to be dict type, it was not possible to pass dict inside the config file. This is due to the fact that you do not need to use solid_properties for St. Venant Kirchoff model, but is required for other material models.

With this change, you can create dict inside the config file as follows

[solid]
solid_properties : {   
    material_model: MooneyRivlin,
    rho_s: 1.0E3,
    mu_s: 0.1724E6,
    lambda_s: 1.55E6,
    C01: 0.02E6,
    C10: 0.0,
    C11: 1E6
    }

[fluid]
fluid_properties : {
    dx_f_id: 2,
    rho_f: 1,
    mu_f: 1E3
    }

Here, I use section heading [solid/fluid], but you just need at least one section heading for new version to work.
A major change from previous version is that now we do not use ’' for str type variable.
For example, previously we had

# Turn on/off solving of the fluid problem ('fluid', 'no_fluid')
fluid="fluid"

but now it should be

# Turn on/off solving of the fluid problem ('fluid', 'no_fluid')
fluid=fluid

but everything else should work as before. Could @johannesring please check this ?