FEATURE: adding explicit method for black scholes

[chemardes]

The following implementation only takes into account a european call option

Changes:

• Implemented the explicit method on the black-scholes equation

Things to do:

• Need to implement it to allow toggling between call/put options (user can set it in the input parameters? e.g. "call"/"put")
• Need to add check to see if user-defined conditions fall within the CFL condition
• Need to add parameter validation (e.g. nodes need to be positive integers)
• Error handling
• Try to implement it in Crank Nicolson
• Add comments