From C++ static_cast to ctor-like conversion

[hollasch]

Instead of static_cast<double>(x) switch to double(x) for easier readability for non-C++ readers and simple for C++ readers alike. In the end, these are the same thing.

Resolves #1222

[armansito]

Thank you for the clean up, I think this is a good improvement overall.

P.S.:
Something occurred to me while looking through the diff (though I don't want this to block this PR): the code examples currently use auto frequently. Sometimes this is good for brevity (for example with gnarly template expansions) but there are a few spots where I think auto obscures the code a little bit.

For instance, from src/TheNextWeek/perlin.h:

auto u = p.x() - floor(p.x());
auto v = p.y() - floor(p.y());
auto w = p.z() - floor(p.z());
auto i = int(floor(p.x()));
auto j = int(floor(p.y()));
auto k = int(floor(p.z()));

The types should generally be clear if you read it carefully but I think it would be clearer with explicit type annotations, especially given the intentional type conversions involved:

double u = p.x() - floor(p.x());
double v = p.y() - floor(p.y());
double w = p.z() - floor(p.z());
int i = int(floor(p.x()));
int j = int(floor(p.y()));
int k = int(floor(p.z()));

Similarly, in src/TheRestOfYourLife/camera.h the assignment to viewport_height relies on implicit type deduction, which should be a double but this may not be immediately obvious upon reading. I think it would be clearer to express that line as:

double viewport_height = 2 * h * focus_dist;
double viewport_width = viewport_height * (double(image_width)/image_height);

Just food for thought.