Sequences restructuring

[Pwhsky]

WIP of restructuring sequences.py and SequentialProperty.

Changes to SequentialProperty:

• SequentialProperty now has two new methods:

set_sequence_length(), set_sequence_step(), which lets you directly set the Property.sequence_length and Property.sequence_step attributes without having to use SequentialProperty.store().

• SequentialProperty now has one new optional argument:

current_value, which is assigned to the SequentialProperty.current attribute upon calling __init__().

Changes to Feature:

to_sequential() is a new Feature class method intended to replace the Sequential() method found in sequences.py.

Sequential() is used as:

rotating_ellipse = deeptrack.sequences.Sequential(rotation=get_rotation)
imaged_rotating_ellipse = optics(rotating_ellipse)

The new Feature method to_sequential() is used as:

rotating_ellipse = ellipse.to_sequential(rotation=get_rotation)
imaged_rotating_ellipse = optics(rotating_ellipse)

[BenjaminMidtvedt]

Should be self

[Pwhsky]

corrected.