Rename concat to concatenate

[aniketsharma00411]

The naming scheme of Matrix methods should be similar to that used in Python NumPy. Accordingly, concat method should be renamed to concatenate.

Steps to follow:

1. Rename the method in matrix_operations.hpp
2. Update the README.md accordingly
3. Rename examples/concatenation.cpp to examples/concatenate.cpp and update the example accordingly.
4. Rename benchmark/BM_concat.cpp to benchmark/BM_concatenate.cpp and update the benchmark to use new method name.
5. Delete benchmark/linux_binaries/BM_concat and generate new binary using g++ benchmark/BM_concatenate.cpp -std=c++11 -I./include/ -isystem ./lib/benchmark/include -L./lib/benchmark/build/src -lbenchmark -lpthread -o ./benchmark/linux_binaries/BM_concatenate
6. Rename BM_concat_row and BM_concat_column methods in benchmark/BM_all.cpp to BM_concatenate_row and BM_concatenate_column respectively and update the method call according to the new name.
7. Recompile benchmark/BM_all.cpp using g++ benchmark/BM_all.cpp -std=c++11 -I./include/ -isystem ./lib/benchmark/include -L./lib/benchmark/build/src -lbenchmark -lpthread -o ./benchmark/linux_binaries/BM_all

Note: All the above steps should be a part of one commit with the message "Renames concat to concatenate".