Method set_limit_side() defined for all descendants of FieldCommonBase.
Fixed MultiField::set_time().
- ensure that set_time() is called for every sub-field
Units passed to subfields in MultiField::init().
New output for TransportDG (WIP).
- initial state is not saved (needs fix in HC_Explicit_Sequantial::run_simulation()
- key "conc_mobile_p0" in TransportOutput record should be modified
Method Input::Type::Record::copy_keys() accepts reference to constant record.
This allows to pass temporary objects.
Fixed missing return value in FieldSet::make_output_field_keys().
Merge branch 'JH-output' into JS_output

Conflicts:
	src/fields/field_set.hh
Added method FieldSet::output_type().
- to set output_type of each sub-field
Name of sub-field in MultiField changed (useful for field names in output files).
Fixed bugs in output.
- missing spaces in VTK output
- wrong cast in OutputTime::compute_field_data()
OutputTime::register_data() checks equality of output_stream given by the record and the field key.
- this is temporary solution until we change the input format so that the fields for output are specified in an array, e.g. output_fields = ["pressure_p0", "cross_section"]
TransportDG with new output.
- output stream and fields are specified in the record "output" inside secondary_equation
- support for writing input fields
New output for ConvectionTransport.