`readfile`: support GDAL `int8` dtype

[yunjunz]

Description of proposed changes

This PR adds the support of reading int8 type for files in GDAL format, as suggested by @scottstanie in #1081.

• add the correction int8 conversion between GDAL and NumPy in DATA_TYPE_GDAL2NUMPY/NUMPY2GDAL

• add numpy_to_gdal_dtype() from dolphin

• add gdal_to_numpy_dtype() from dolphin

I did not replace the usage of dict conversion with the two functions above yet, because cint16/32 is supported by GDAL but not in numpy yet, thus, the dict conversion may be more generic at the moment for translation purposes.

Reminders

• [x] Fix #1081
• [x] Pass Pre-commit check (green)
• [x] Pass Codacy code review (green)
• [x] Pass Circle CI test (green)
• [x] If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.