Eradicate the "ignoring attributes on template argument" warning

At least since 1.2.1 compilation on linux with glibc results in a warning being emitted about an attribute being discarded when decltype(&PCLOSE) is passed as a template argument to std::unique_ptr:

/home/masflam/Clones/matplotplusplus/source/matplot/util/common.cpp: In function ‘std::string matplot::run_and_get_output(const std::string&)’:
/home/masflam/Clones/matplotplusplus/source/matplot/util/common.cpp:54:48: error: ignoring attributes on template argument ‘int (*)(FILE*)’ [-Werror=ignored-attributes]
   54 |         std::unique_ptr<FILE, decltype(&PCLOSE)> pipe(POPEN(cmd.c_str(), "r"),
      |                                                ^

This is because pclose is declared with the attribute __nonnull((1)) in glibc headers:

/* Close a stream opened by popen and return the status of its child.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int pclose (FILE *__stream) __nonnull ((1));

And due to -Werror the whole compilation fails. The simple fix in this PR is to pass the deleter's type explicitly, since barring attributes PCLOSE's signature is the same on all platforms.

Fixes #403

alandefreitas / matplotplusplus

Eradicate the "ignoring attributes on template argument" warning #423