Bring loading methods inside the class `ImageDataCreator`

[DanicaSTFC]

Bring loading methods, such as

iDVC/src/idvc/io.py

Line 635 in 2410e7d

def loadTif(*args, **kwargs):

or

iDVC/src/idvc/io.py

Line 531 in 2410e7d

def loadNpyImage(**kwargs):

(and related dependent methods) inside the class

iDVC/src/idvc/io.py

Line 51 in 2410e7d

class ImageDataCreator(object):

See #387 to see how this was done for loadNxS method.