Find conda-installed binaries on Windows

[willGraham01]

niftyreg backend binaries

Addresses a difference between Windows and Mac/Linux OSes when attempting to locate CONDA-installed niftyreg binaries.

Addresses #116, but does not close it.

Changes

If the user has installed niftyreg into a conda environment, then installed brainreg into the same environment, we should detect the conda-installed niftyref binaries and use these in the backend rather than the binaries that we bundle with the package.

Previously, Windows users experienced an error due to the conda install path of the niftyreg binaries being different to that on Linux/MacOS. Full changes are:

• .exe is now explicitly appended to all executables/binaries when on Windows OS
• The conda install path is properly set when the underlying OS is Windows (CONDA_PREFIX/Library/bin vs CONDA_PREFIX/bin on Mac/Linux)
• Some backend tests have been added along with an additional step in the CI to confirm that package installs now correctly detect when there are CONDA-binaries that can be used.