Copy constructor bugfix release

This release fixes the broken copy-constructor assignment pointed out by @lukasberbuer and a lot of smaller fixes by many others in particular for the cmake file but also missing default constructors so that iir library instances can be re-assigned without the need of new/delete which is the way forward. There is a new unit test which also tests the copy-constructor.
There are now options to disable exceptions, the unit tests and the demo programs.

Fixed pkgconfig and cmake-config for MacOS

This release has now a working packageconfig thanks to @eli-schwartz and for Mac users the find_package() function should now provide also the library path when using iir::iir for the dynamic library.

Minor fixes and C++11 compliance

Added default constructors and default initialisers for all classes.

Minor fixes

Small bugfixes to increase compatibility and an option to disable exception handling (see the readme).
More unit testing for the state classes.

Normalised frequency setup functions

I've added setupN() functions for normalised frequencies for pure sampled systems with no real world sampling rate.
Normalised frequencies range from f=0..1/2 which means that the highest frequency is represented by two samples (i.e. +1,-1 or scaled/shifted). The unit is just 1/samples.
One smaller bugfix and more unit tests.

Performance enhancements with C++11 language loop features

Filtering is performed by sending the sample through a chain of 2nd order filters. These are now iterated using the C++11 language features to loop through them so that the compiler has more options to optimise it. Try -O3 and/or -march=native when compiling on your local computer. All filtering operations are header only so that they are directly compiled into your code.

Support for FetchContent and simplified custom coefficients

@epezent has sent a pull request so that the filter works with FetchContent.
Custom coefficients can now be directly set in the constructor so no "setup()" is needed. Of course setup() can still be used for setting the coefficients if needed.
Another unit test has been added for the custom coefficients.

Bugfix release

The range check for the cutoff frequency included Nyquist but because the filter does a pre-warp with tan(pi * fc/fs) that leads to unpredictable results. Nyquist is now excluded to prevent this.

Minor bugfix release (Mac)

Max Thrun (@bear24rw) provided fixes for the Mac compilation and Sven Herrmann pointed out that some files had Windows and some other Unix line endings which have been fixed. clang also complained about not running in C+11 mode which has been added to CMakeLists.txt to enforce it.

Bugfix release

Fixes a bug where the output resulted into zero if the order in setup() was lower than the order in the template argument.
Thanks to Thomas Gulde who pointed this out!