Added exponentiation operator and cleaned up the code.

...t/cs/gitlab/maikov/exceptions/InvalidMatrixException.java

.../cs/gitlab/maikov/exceptions/InvalidOperandException.java

...s/gitlab/maikov/exceptions/InvalidReductionException.java

...va/ee/ut/cs/gitlab/maikov/expressions/Exponentiation.java

...n/java/ee/ut/cs/gitlab/maikov/expressions/Expression.java

src/main/java/ee/ut/cs/gitlab/maikov/expressions/MathOp.java

...a/ee/ut/cs/gitlab/maikov/expressions/MatrixInversion.java

.../java/ee/ut/cs/gitlab/maikov/expressions/MatrixValue.java

src/main/java/ee/ut/cs/gitlab/maikov/expressions/Number.java

...ain/java/ee/ut/cs/gitlab/maikov/expressions/Tehtepuu.java

...in/java/ee/ut/cs/gitlab/maikov/expressions/Vahendaja.java

src/test/java/ExpressionTest.java