Add Shape Function definition methods
Add Shape Function definition methods

Added the following methods:
get_inv_Me()
get_Ne()
get_Be()

These were based on the Element1D class method and appropriately changed for the Element2D class (only minor changes were required).

self.p, self.Me and self.Ne were also defined as 'None' in the __init__ in order to assess if their respective methods have been called in the program yet.
MrJarv1s/FEMur