Fix parser

Fixes #445 (closed).

I am using a new variable in the parameter structure (is_default) that I set to 1 if we write in params a default value.

Currently I am only checking on the get_opt_params function. I am wondering if we should ensure that we are not reading a mandatory parameter from a default value.

For example:

parser_get_opt_param_double(params, "Test:test", 1.); // Test:test does not exist in params
parser_get_param_double(params, "Test:test"); // Here currently we read the previous default value without any warning

Any suggestion @pdraper, @matthieu?