Str eat dev

* Broke long sentences per paper into bullet points for notebooks
* Added note about downloading notebooks
* Added note about adding ones own paper
* Added note for citing pycroscopy

Tested on Force spectroscopy and fixed few bugs

Renamed h5_mean_spec dataset to "Mean_Spectrogram" to prevent conflict with existing 'Mean_Ronchigram' dataset.

Fixes some small bugs found during test creation

Now if a note value is not found a NameError is raised.
Instrument model, translator, and Asylum Research software version are
present as attribute at the root of the hdf5 file produced.
Position indices are correct.

Error message changed to refer to get_component_slice rather than clean_and_build

First working translator.

This is the first version of the "AsylumResearch hdf5" to "pyUSID hdf5"
translator. It was tested only on a couple of files, so more testing is
required. Also it is necassary to double-check that Ancillary dataset
(especially Position_Indices) are correct.

New Translator extends the pyUSID ImageTranslator to allow for reading dm3 and dm4 files.  No changes should be necessary for existing code.

Str eat dev

Ensure that current_data_exists, exists