Add generic file import

[tomjholland]

This PR:

• Adds an import option to cell for importing a generic file (i.e. not a specific cycler format)
• Modifies the process for importing cycler files by not inferring the schema on the scan, schema is instead enforced when the data is converted later
• Simplifies unit class to deal primarily with PyProBE data format
• Simplify cycler classes to remove column_name_pattern, common_prefix and common_suffix attributes
• Cycler column dict rearranged to be Cycler format: PyProbe format
• Cycler column dict includes asterisks to indicate presence of units