FIX: Form factor calculation bugfix (input dtypes)

Fixed an issue where using a combination of dtypes for the input hkl tuple would result in ValueErrors.

Changed the method of checking dtypes and calculating the initial zero value for the loop over each atom.
Merge branch 'structure-factor-bugfix' into develop
REF: Moved load_file from Data class to separate method

load method now creates a data class and supports 'auto' detection of filetype.

Also some basic refactoring to reduce line lengths.

Tests and docs updated to reflect changes.
ADD: Load and save methods

Added load and save methods to create Data object from file or list of files.

Includes auto-detect data type.
Fixed docstrings.
Removed load_file from Data class definition.