Move scan calculations out of nexgen

[DominicOram]

As a user of nexgen I would like to provide the scan details myself rather than the current system of the scan being assumed as part of the goniometer structure. I propose that we provide this using scanspec as it's an existing library which makes it easy to construct scans.

Nexgen should probably still be responsible for creating the additional omega scan values (when none are provided) as this is a quirk of the nexus structure that omega values are always required so shouldn't be exposed to the user.

Acceptance Criteria

• nexgen can be called with a full scan specification and will write this
• The detail around the scan in the gonio structure (e.g. increment, start and end) becomes optional
• nexgen will create the omega scan part if this hasn't already been provided
• There are unit tests covering all the above

Note

• Currently i24 still relies on nexgen to build the scan, if this is still the case when this ticket is done then create a new one for removing the scan generation code once no longer required by i24

[noemifrisina]

First part of the work done in #110 and Artemis#599. Scans are calculated and passed directly from artemis. End and increment positions are now optional, but for the moment the start position is not.
Merging #110 to get this first change working. There's still some work to be done on creating omega if not passed (and some tests to be added) but that will go in another PR.