Adding the ability to learn multiple skills simultaneously

[niklub]

The idea of this PR is to specify the linear skill set and to make it trainable on the basic ground truth environment, for example:

from adala.agents import Agent
from adala.skills import LinearSkillSet

agent = Agent(
    skills=LinearSkillSet(skills=[
        'Identify keywords that best describe the meaning of the text',
        'Generate new text based on the keywords'
    ])
)

agent.environment = BasicEnvironment(ground_truth_dataset=Dataset(df=df))
agent.learn()