Resolve depreciation, change np.float64 to float

[ICWallis]

This PR addresses #50 by changing np.float64 to float, as suggested by the Numpy release documentation https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations .

[ICWallis]

I ran the tests and got the following error. I was expecting the tests to run the local version of the code, rather than the installed package. Some suggestions for how to test this change would be appreciated.

(mps_env) PS C:\mplstereonet>  & 'C:\Users\Irene\miniconda3\envs\mps_env\python.exe' 'c:\Users\Irene\.vscode\extensions\ms-python.python-2023.16.0\pythonFiles\lib\python\debugpy\adapter/../..\debugpy\launcher' '53481' '--' 'C:\mplstereonet\tests\test_stereonet_math.py' 
Traceback (most recent call last):
  File "C:\mplstereonet\tests\test_stereonet_math.py", line 2, in <module>
    import mplstereonet.stereonet_math as smath
ModuleNotFoundError: No module named 'mplstereonet'
(mps_env) PS C:\mplstereonet>