Please clarify license, and add SPDX identifiers

[StefanBruens]

As stated in the License file, each source file should specify its appropriate license.

Notably, it should also specify which version of the LGPL should be used, in this case either LGPL-2.1-only or LGPL-2.1-or-later.

OndselSolver/LICENSE

Line 147 in 7371522

To apply these terms, attach the following notices to the library. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.

Preferably, each file also has a SPDX license identifier, see e.g. the FreeCAD sources, where each source file starts with something like:

// SPDX-License-Identifier: LGPL-2.1-or-later