funtion creating vector<vector<T>> from istream

[Peter]

In Vector and Matrix the istream constructors contain code loading a vector<vector<double> > from the istream. We should lift out this to a function.

1. This will avoid duplicating code.
2. The function might be useful elsewhere.

There is no reason to limit to double, so why not allow types such as int and string as well.

[Peter]

(In [1922]) working on ticket 510 (refs #510)

[Peter]

(In [1923]) refs #510 - avoid problem with empty lines

[Peter]

(In [1927]) refs #510 using load function in Matrix istream constructor. Behavior should be as before except that when Matrix is invalid the error message no longer contains info on which row and column that failed.

[Peter]

(In [1928]) refs #510 - allow function 'load' to ignore empty lines and check that columns contains equally many columns. remove corresponding code in Matrix.

[Peter]

(In [1929]) refs #510. using load function in Vector(istream) constructor

[Peter]

(In [1930]) avoid copying in MatrixWeighted??(istream), i.e., avoid use of intermediate Matrix. closes #510