Check mesh compatibility from field_expression

[gouarin]

When we try to build a field from a field_expression, we don't check whether all the meshes in the fields making up this expression are compatible.
And, to build the mesh of the output field, we take the first find in the expression.

see https://github.com/hpc-maths/samurai/blob/master/include/samurai/field.hpp#L546