Escape XML responses

[matprov]

Description

XML responses should be escaped, in order to expose valid XML.

For example, in a service that uses pywps package, we currently get this response when querying ?request=GetCapabilities&service=WPS:

...
    <ows:Title>Maximum number of consecutive frost days (Tn < 0).</ows:Title>
...

To be valid, the response should be:

...
    <ows:Title>Maximum number of consecutive frost days (Tn &lt; 0).</ows:Title>
...

[cehbrecht]

I have added an example process to Emu with characters that should be encoded: https://github.com/bird-house/emu/pull/115

The encoding of < and > is working in my local test with pywps 4.4.5.