Add support for software field

This allows to set the software field in the message sent. And makes the instrumentname optional, as we do not need it all the time.

For example this call:

client = IcatElogbookClient(
    "https://...", type="broadcast", source="ebs", tags=["testtag", "machine"], software="defaultsoft"
)
client.send_message(
    "mycontent", category="comment", tags=["commenttag"], software="mysoft"
)

sends this message

{'apikey': ...,
 'category': 'comment',
 'content': [{'format': 'plainText', 'text': 'mycontent'}],
 'creationDate': '2022-05-13T14:56:05.063887',
 'machine': '....esrf.fr',
 'software': 'mysoft',
 'source': 'ebs',
 'tags': [{'name': 'testtag'}, {'name': 'machine'}, {'name': 'commenttag'}],
 'type': 'broadcast'}