[#43475101] Added Gel class implementation
[#43475101] Moved common attributes to container

Moved plate's, tube rack's and gel's common
attributes to container module.
[#43475165] Implement persistence for Gel class
[#43475341] Implement create action for Gel resource
added missing newline to the end of file(s)
[#43475341] separate methods for gel/plate creations
[#43475165] added Container and containerElement mixing

Refactored common code from plate and gel persister
to mixing and include them into the relevant classes.
[#43475165] add Container/ContainerElement mixin to sequel

Refactored common code from plate and gel persister
from the sequel namespace to mixin
and include them into the relevant classes.
[#43475341] moved common gel/plate code to a mixin
[43475101] Use the correct variable naming conventions

Changed some variable names from camelCase to
snake_case and renamed some variables.
Merge pull request #118 from ke4/43475101_implement_gel_class

43475101 implement gel class