Add proper Tsodyks-Markram STP mechanism implementation.Not ready yet.

As can be seen from Ex7 (especially evident in the second pulse), the
state discontinuity A = A + plasticityFactor * factor seems to use the
"plasticityFactor" value from the previous timestep. This has nothing
to do with having plasticityFactor entirely calculated by a child
element, as one can get the same effect by appropriately changing the
expressions for A, B and g in the existing stpSynapse componentType.
Fix Tsodyks-Markram STP mechanism.

Now tsodyksMarkramSTPSynapse should have the desired behavior (compare
"tmStpSyn" with "stpSynDepFac" in Ex7). The old, broken,
model (stpSynapse) is still present to allow for a comparison.
Tidy up: remove unneeded variable in STP synaptic mechanism.
Separate types for facilitating and non-facilitating TM mechanisms.

A component of type tsodyksMarkramSTPSynapse can now have a different
behavior (depressing and facilitating or depressing only) depending on
the (sub)type of its child element plasticityMechanism.
Exponential synapses: only calculate waveform shape parameters once.

In the previous implementation, "peakTime" and "factor" were being
recomputed at each timestep despite being constant.
Remove old stpSynapse model. Rename new one to take its place.
Small variable name changes.
Add <notes> child element for all cell and synapse component types.
Merge branch 'master' into stp
git: ignore build directory.
Merge branch 'master' into stp
Remove misplaced parameter from nmdaSynapse.

Also, rearrange element declaration ordering in Synapses.xml to make
the file more compliant to the LEMS schema. There is still an issue
with the use of DerivedParameter, though, which has been recently
removed from the schema, even though it may be reintroduced soon.
Update schema and examples with new stpSynapse and blockStpSynapse.
Merge branch 'unittest' into stp
Rename stpMechanism to plasticityMechanism, and so on.
Merge branch 'master' into stp

Conflicts:
	Schemas/NeuroML2/NeuroML_v2beta.xsd
Fix typo in plasticSynapse definition.
Update STP example (Ex7) to use plasticSynapse instead of stpSynapse.
Rename stpSynapse -> plasticSynapse in schema.
Rename stpSynapse -> plasticSynapse in NML2_SynapseTypes example.