ENH: Replace 'char' with int8_t when numeric is intended

[hjmjohnson]

The 'char' type can be signed or unsigned on different platforms
and is generally used for ASCII character storage. To conistently
represent 8-bit signed values on differnt platforms the int8_t
type is recommended.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)

[N-Dekker]

I would suggest using signed char, rather than int8_t, in a context that also deals with unsigned char in a similar way. However, that's no show-stopper to me. Just something to consider...

[hjmjohnson]

I would suggest using signed char, rather than int8_t, in a context that also deals with unsigned char in a similar way. However, that's no show-stopper to me. Just something to consider...

@N-Dekker

This PR was intended to take the first step towards some loose best-practices conventions scattered about stackoverflow/reddit/other sources:

• Never use 'char' unless explicitly desire to work with old API's or intending characters (i.e. ASCII). The 'char' type has a different meaning based on compiler settings or platform defaults.
• Allow 'unsigned char' and 'signed char' when the intent is bytes (i.e., for bit encodings, or enumerations).
• Prefer 'uint8_t' and 'int8_t' when the data storage is intended to have numerical properties (addition, subtraction etc...)

In the instances you gave, I would prefer to use 'uint8_t' instead of 'unsigned char'.

Given your statement that the current is OK, I am going to leave this PR as is and submit another PR in the near future with more instrumentation of 'int8_t' and 'uint8_t'.

NOTE: I am teaching a graduate-level course that includes an important component on teaching best practices using ITK examples. My personal preference for int8_t and uint8_t is that they more clearly demonstrate to new developers (many of whom are first-time C++ programmers) that these are just very small integer data types. It is easy for these new programmers to understand that int8_t is an 8-bit integer.

[N-Dekker]

Thanks for explaining, Hans.

• Allow 'unsigned char' and 'signed char' when the intent is bytes (i.e., for bit encodings, or enumerations).

When the intent is just raw bytes, it might be preferable to use std::byte, as introduced with C++17 😇