fix invalid supported UOMs description

[fmigneault]

Overview

Fix process description literal UOMs reporting duplicates of the first default UOM instead of all supported UOMs.

For example, passing uoms=['metre', 'feet'] to LiteralInput would cause it to report as follows in the WPS description.
The default UOM would be repeated for as many supported UOMs that were passed to the input.

<LiteralData>
  <ows:DataType ows:reference="http://www.w3.org/TR/xmlschema-2/#float">float</ows:DataType>
  <UOMs>
    <Default>
      <ows:UOM ows:reference="">m/s²</ows:UOM>
    </Default>
    <Supported>
      <ows:UOM ows:reference="">metre</ows:UOM>
      <ows:UOM ows:reference="">metre</ows:UOM>  <!-- error should be 'feet' -->
    </Supported>
  </UOMs>
</LiteralData>

At the same time, this fixes the optional reference attribute accordingly if it was not provided explicitly, or could not be mapped to a known unit of OGC URN.

According to https://schemas.opengis.net/ows/1.1.0/owsDomainType.xsd
the ows:UOM which depends on ows:DomainMetadataType
shows <attribute ref="ows:reference" use="optional" />

But the previous check of UOM class caused it to check of OGCUNIT if not provided with an empty string.
The corresponding if in the template were also not consistent about reporting reference.

Related Issue / Discussion

Required by crim-ca/weaver#541
Required by crim-ca/weaver#430
Fixes #685

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute bugfix to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[cehbrecht]

@fmigneault with PR #687 the tests should work again. If you think this PR is ready I can merge it.

[fmigneault]

@cehbrecht
Can you consider this fix into 4.6.0 release?

[fmigneault]

On my side, the tests were running, but you can combine them with the other PR to validate they work together.
Thanks!