PiecewisePolynomialDiscontinuousScalarSpace now handles correctly segments
with polynomial order = 0 (piecewise constants).
Added strictlyOnSegment to PiecewisePolynomialContinuousScalarSpace.
Reduced size of mesh used in tests of piecewise polynomial spaces.
Added new tests of PiecewisePolynomialDiscontinuousScalarSpace.
Added new tests of PiecewisePolynomialContinuousScalarSpace.
Improved documentation of some spaces.
Extracted DofAssignmentMode into a separate header.
Corrected a bug in PiecewiseLinearContinuousScalarSpace (with segments).
PiecewisePolynomialDiscontinuousScalarSpace supports now multiple dof assignment modes.
Added an example of solving a Laplace problem with mixed BCs using
higher-order basis functions.