Fix an "implicitly-declared copy assignment operator" warning

[ssilverman]

Discovered with -Wextra (-Wdeprecated-copy).

Addresses part of #660.

I think I've done this correctly, but please triple-check my changes. (@luni64 I'd love your opinion here too.) I believe the behaviour changes because, when it uses the implicitly-declared copy assignment operator, copy_tcd() isn't called, but with my changes, it is called.

It's this function that's complaining:

DMASetting(const DMASetting &c) {
  TCD = &tcddata;
  *this = c;  // <-- Over here
}

[ssilverman]

Making this a draft for now.

[ssilverman]

This commit also makes sure each copy constructor has a sibling operator=().

[luni64]

Here is my assessment as requested by @ssilverman:

By default, if not provided or deleted by the user, the compiler automatically generates both a copy constructor and the corresponding assignment operator. If the class has a user-defined copy constructor but no user-defined assignment operator, auto-generating the missing assignment operator is considered 'dangerous' and is deprecated.

This results in the following very explicit warning message:

.../DMAChannel.h: In copy constructor 'DMASetting::DMASetting(const DMASetting&)':
.../DMAChannel.h:524:25: warning: implicitly-declared 'DMASetting& DMASetting::operator=(const DMASetting&)' is deprecated [-Wdeprecated-copy]
  524 |                 *this = c;
      |                         ^
.../DMAChannel.h:521:9: note: because 'DMASetting' has user-provided 'DMASetting::DMASetting(const DMASetting&)'
  521 |         DMASetting(const DMASetting &c)
      |         ^~~~~~~~~~

Since deleting the assignment operator is not an option here, one should define a corresponding user assignment operator as @ssilverman proposes. I also doubt that the auto-generated assignment operator does perform a proper copy_tcd(), I assume it does some bit-wise copying of the data only.