Issue at corner node when a variable of TIME_INDEPENDENT equation type and some boundary conditions

[david-montiel-t]

Whenever NATURAL or PERIODIC boundary conditions are used for a variable of TIME_INDEPENDENT equation type, the (0,0) corner node is "pinned" to zero if mesh adaptivity is enabled.

[david-montiel-t]

The pinning at the corner has been disabled for variables associated with TIME_INDEPENDENT equation types. This means that the code may not be able to find a unique solution for TIME_INDEPENDENT equations that include only spatial derivatives of the variable if no DIRICHLET boundary conditions are present. A functionality to handle this particular case should be implemented.