FEATURE: implementing solver classes for python lib

[chemardes]

The following PR addressing Issues #3, #4 , #7

Idk if this is the best way to do it , but this was my interpretation of using classes for the python library, comments are welcome!

@alphaville, @debdal6

Changes:

• Utilised classes to format the code
• Made use of the Strategy Design pattern (https://refactoring.guru/design-patterns/strategy/python/example) to dynamically change the methods used to solve the equation
• Changed the way I calculated dx and dt (used Debdal's approach as it's much cleaner)
• Fixed initial and boundary conditions

Things to do:

• Need to add comments
• Need to implement error handling
• Need to add tests (checking the error between methods + unittests)
• Maybe make a utils file?