Implement input validation

[crybot]

As of now, there is no input validation implemented. A possibly incomplete list of validation requirements is as follows:

• All currently implemented textfields should be validated as non-negative integer inputs and should not be empty

• The length of the microcycle should not exceed 30 days and should be a multiple of 7 days

• The number of rest days should not exceed the length of the microcycle

• The given maximum value of consecutive workouts should no exceed the length of the microcycle

• The given maximum value of consecutive rest days should no exceed the length of the microcycle

• Each rotation input should be an integer in the interval [0, 3]

• Other inputs' constraints are implicitly enforced by the optimization procedure, so there is no need to manually specify them.

Input validation should be performed prior to split generation and its outcome should be communicated to the user somehow

[crybot]

Closing after commit 6fd0fcd5ec4aa88c83c94e7b9bea3fd9aff33380