Replace LastModifiedDate in IDF with a checksum

[NickDraper]

This ticket is blocks : TRAC11565, TRAC11818

The lastmodified date in the IDF is a poor proxy for a checksum and in only used in mangling the instrument name in the IDS.

Now we have a ChecksumHelper we should use that, create a SHA1 or MD5 and use that instead in the IDS name.

1. Change the name mangling to use the checksum instead (including for text in nexus files).

2. Change the IDF files to remove the date

3. Change the concepts docs that mention this

4. Make sure the SNS IDF generator scripts no longer include this

5. Change the xml schema to remove the need for this field.

   Keywords: MAINTENANCE

[NickDraper]

This issue was originally trac ticket 11569