julia/MOLE.jl: Add addScalarBC, relative BCs tests and elliptic1DaddScalarBC example

[valeriabarra]

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Example
• Documentation

Description

This PR adds a new set of Boundary Conditions in Julia. Specifically, it adds:

• the AbstractScalarBC abstract type (so that we can generalize to multiple dimensions later)
• the ScalarBC1D concrete type and convenience constructor
• a couple of helper functions
• the addScalarBC! functions to apply the scalar type BCs
• unit tests for this type of BCs (not present in the MATLAB/Octave counterpart)
• translation of the elliptic1DaddScalarBC.m example as elliptic1DaddScalarBC.jl

Related Issues & Documents

• I have created an Issue that is paired with this PR
• Closes Translate the addScalarBC1D Boundary Condition and add an example that shows its usage (elliptic1DaddScalarBC) #299

QA Instructions, Screenshots, Recordings

Added/updated tests?

We encourage you to test all code included with MOLE, including examples.

• Yes
• No, and this is why: please replace this line with details on why tests
  have not been included
• I need help with writing tests

Read Contributing Guide and Code of Conduct

• 📖 I have read the MOLE Contributing Guide: https://github.com/csrc-sdsu/mole/blob/main/CONTRIBUTING.md
• 📖 I have read the MOLE Code of Conduct: https://github.com/csrc-sdsu/mole/blob/main/CODE_OF_CONDUCT.md