Handle UWS XML quote negative values

Currently, negative quote values do not work.

UWS 1.1 specifies 'nil' value should be used for an unspecified quote value, but UWS 1.0 allows negative numbers too:

UWS 1.1 (http://www.ivoa.net/documents/UWS/20161024/REC-UWS-1.1-20161024.html#Quote)

> Quoting for a computational job is notoriously difficult. A UWS implementation should provide a quote object, in order that the two-phase committal of jobs be uniform across all UWS, but it may supply a **“don't know” answer** for the completion time, **indicated by an XML xsi:nil element** in an XML representation of the quote object.

UWS 1.0 (http://www.ivoa.net/documents/UWS/20101010/REC-UWS-1.0-20101010.html#Quote)

> Quoting for a computational job is notoriously difficult. A UWS implementation must always provide a quote object, in order that the two-phase committal of jobs be uniform across all UWS, but it may supply a **“don't know” answer** for the completion time, **indicated by a negative value or an XML nil element** in an XML representation of the quote object.

I think that, in order to ensure UWS compatibility, negative numbers should be allowed.

No file or test is named in the issue. Locate the UWS quote handling entry point and its tests, then verify how negative XML values are parsed or serialized; done means UWS 1.0 negative quote values work without breaking the UWS 1.1 nil representation.

Written by the indexing model from the issue text.