Accept a Python sequence instead of an R3 instance

A SWIG typemap is added to map a Python sequence to an R3 instance (Wrap/Swig/r3_typemap.i).

This change allows the following Python code, for instance:

import bornagain as ba

sample = ba.Sample()
v0 = (1, 2, 0)
sample.setExternalField(v0)

while the C++ signature of Sample::setExternalField is

void setExternalField(const R3& ext_field);

Note that this method works only for functions or methods which are not overloaded. Overloaded functions or methods must be explicitly 'extended' in SWIG to accept a Python sequence instead of an R3 instance. See https://www.swig.org/Doc1.3/Typemaps.html#Typemaps_overloading.

Additionally, the code is improved to avoid copying R3 arguments when possible. A const reference is used instead.

Related to issue #111