Fix remaining issues in single precision mode

[denisalevi]

The test suite in single precision mode has a few tests that sometimes fail, mostly due to tests not taking into account precision. This issue collects the issues related to single precision test fails and other single precision improvements.

• Make single-precision version of exprel and clip typesafe #233
• Single precision f-literal replacement fails for variables with double digit + dot in name #254
• Make inverted assertion tests single precision compatible #262
• Check if single precision math functions are used correctly  #263
• This test appears to fail when run in the test suite but not when run outside the test suite (in single precision mode): test_stdp_heterog_delays_example