Allow different numbers of frames per dataset

[DominicOram]

Currently nexgen assumes that the Eiger writes out 1000 frames per dataset. This is actually configurable and we would like to configure it (see https://jira.diamond.ac.uk/browse/I04-1043).

Acceptance Criteria

• When I create a NXmxFileWriter I can specify how many frames I expect to be in each dataset
• Nexgen will create a nexus file with VDS to the correct correct shape for this number of frames

[noemifrisina]

Small note. This is already done in nexgen for electron diffraction datasets collected on a Singla detector, we can probably reuse that logic. (will dig out code)